And so the big day arrives.

Start the day at 6.30am. Not hungry at all but stuff my face with bananas on toast and a couple of cereal bars.

I don my London ‘A-Z’ shirt and pin on my number for the day 15673. After checking my official Flora kit bag for Vaseline, Lucozade, banana, sun cream, water and more cereal bars, I head off for Surbiton station to catch the eight o’clock for Waterloo.

Not surprisingly as I board the train there are several other marathon entrants, plastic kit bag in hand, excited supporters in tow, all talking about the day ahead and, the weather.

Although I’m feeling confident and determined to enjoy the day the threat of temperatures reaching the mid-20s is a serious worry.

Along with several hundred other runners, I cross to Waterloo East and join an absolutely packed train smelling of bananas and liniment oil. There is already a great buzz of expectation in the air and the beautiful blue sky and sunshine has got everyone in a good mood.

The marathon, like other major events in London, has an amazing affect on people. Normally on a packed commuter train the art is to avoid any personal contact with strangers. This morning however, everyone talks to everyone else, and jokes are flying around the carriage faster than the glucose sweets.

At Blackheath station we all disgorge from the train and make our way in a long crocodile between the columns of fluorescent jackets pointing us towards the heath.

Bathed in sunshine the marquees, baggage trucks, tethered hot air balloons, refreshment trucks create a carnival atmosphere and for as far as the eye can see there are runners removing tracksuits, jogging suits, black bin liners and every other form of outer clothing. Sixty thousand nipples are being greased with Vaseline and muscles are being stretched and pulled in a million and one directions.

As I join the queue for the loo, bottles of suntan cream are being passed backed down the line and everyone’s conscious of the temperature rising.

I join Bay 9 at the Blue Start, only to discover it’s the very last bay, the furthest from the start. In fact we can’t even see the start from where we’re waiting. As I try to keep calm and try to loosen up the muscles for the task ahead I’m joined by two trees, four clowns, a fireman in breathing apparatus, a nun, six people in funny hats and (of course) a rhino! Rather than a marathon, it feels as though I’ve stumbled on some weird fancy dress party on a park common in south London at 9 o’clock in the morning - and I thought this was a serious challenge!

From where we are we don’t even hear the starting claxon. Our only signal that something is happening the other side of the heath is that the thousands of people in front of us start moving forward. Fifteen minutes later, we cross the start line. I’ve already jogged about half a mile and we’ve only just reached the start! Still the micro-chip strapped to my shoelaces will only trigger as I cross the start so at 9.58am I begin to tackle the 26.2miles!

Already the crowds are 10 or 20 deep on either side of the road cheering and hooting. They call out the names written on various parts of the runners' anatomy or shout for the odd passing Viking, Elvis or Wookie. I didn’t write my name on my running gear but already I’m christened ‘map man’ and as the marathon progresses I become Mr A to Z, Az man and a million other variations. Between Blackheath and the Mall I’m asked directions from runners and supporters every 50 yards or so, the joke is funny now but I may have five hours of this.

The heat is already overwhelming as we head across the Sun in Sands roundabout and out of Blackheath towards Woolwich. The crowds remain constant and everyone is in high spirits. At this point I already know I’ll make the finish as long as we all don’t self combust before Tower Bridge.

As the Woolwich ferry comes into view the runners from the green start join us and there is even less room to move. Throughout the entire run you are constantly on your guard, dodging slower runners and trying not to block faster runners coming from behind. Already everyone is trying to cross to the side of the road that has any shade. The difference in the temperature from running in direct sunlight to that in the shade is a good three or four degrees. Consequently you have to choose to run in a mass of people and suffer the body heat or run in direct sunlight.

We eventually reach Greenwich and run the little detour around the Cutty Sark (sadly boarded up for restoration) and head towards Tower Bridge. In theory, my family supporters were going to look out for me at Maize Hill (about six miles). As I’m running, I text them at each mile marker to let them know roughly where I am. As suspected however, and we had all been warned, the mobile phone transmitters in and around the course were at melt down by eight that morning - messages were not getting through. I look out for them but no one's there, my heart sinks. I know the next meeting point is at 16 miles so I settle myself into a steady run, it feels an awfully long way away.

Then comes the first disaster of the day. We reach the first water stop and they’ve run out. The mood of the group of runners I am with suddednly turns nasty and for the first time today there is anger rather than outright happiness among everyone around me. Runners topple over each other as some stop to grab discarded bottles from the floor. There are small amounts of water resting in them as they lay in the gutter. I strike lucky. Thanks go to a little boy in a green t-shirt, probably no more than eight years old. He’s picked up several bottles with small amounts of water in and poured them into one - he sticks his hand out just as I pass and I get a full bottle. I gulp down half the bottle, pour a small amount over my head and pass the bottle to a runner next to me. She does the same and passes it on. It’s gonna be a long hot day.

At six and a half miles I suddenly hear my daughter Jessica’s voice screaming from the crowd. I attempt to cross over and just about touch her as I get sucked past by the mass of runners. My heart leaps. I’ve got water onboard, my body is feeling good and that split second of love from the crowd boosts my confidence.

Mile markers seven, eight, nine go by. The temperature is rising, water stations pass by, some have supplies, some don’t. As we head out of Canada Water, a ripple of excitement is working its way back through the runners, the first set of showers on the course. Although, when I say showers, it’s a covered area with jets emitting a fine spray of water. It’s the best 100 metres so far. Cold water soaks my hair, my shirt, my shorts, my shoes and immediately everyone picks up their pace around me and I do likewise.

The sun is now getting much higher in the sky and there is less and less shade to run in. An increasing number of runners are beginning to walk and dodging in and out is taking up more and more energy and very distracting, it’s hard to keep a steady rythmn going.

Then it happens again. “Go daddy, go!”. My eldest daughter - with boyfriend in tow - are at about 11 miles. This time on the right side of the road and as I give her a ‘high-five’ my heart and head shift up another gear. Her gorgeous smile carries me the next mile or so which is just enough to get me to that first crucial part of the course. We turn right on the course and the road widens and in the distance I can see the turrets of Tower Bridge - that’s nearly halfway.
The crowds are now huge and the noise, the shouts, the cheers, the laughter are totally infectious. The pounding feet, the wobbly bottoms, the sound of 5,000 people breathing heavily around me increases in pace. As we cross the bridge the sound of an amazing jazz-swing band eminates Glenn Miller tunes and London is smiling. The whole of London is smiling!

We take a sharp right out towards Docklands and the halfway point is in sight. My legs feel great, my Achilles is behaving, I’m hot, I’m happy and all those long, long cold runs during January seem a million miles away - the finish not quite so far.

At this point you can actually see the more elite club runners heading back towards Tower Bridge and the finish. Funnily enough it isn’t depressing, which is what I thought I would have felt, it makes me feel that things are gonna be ok. As we pass the 13-mile marker I check the time and I’m doing fine. In fact more than fine - if I can maintain the pace, I will be in the Mall well within my five-hour target!

As the Limehouse tunnel comes into view we do a sharp right and head out towards West Ferry and the roads become much narrower. It’s around 12.30 and it's now overwhelmingly hot. There is absolutely no shade whatsoever and runners begin to keel over and sit by the roadside. Everyone’s pace is definitely beginning to flag. The water stations have less and less water. Because the streets are narrower, there are also less people in the crowd. I pass several runners who are singing at the top of their voices. they have iPods on and are concentrating on running to the beat.

As I continue at a reasonable rate I suddenly hear shouts of "Come-on A to Z" well in front of me. How on earth could they see me coming? Then, horror of horrors, like turning up at some major celebrity function in the same suit as the host, there is a woman 50 yards ahead and she’s wearing the same shirt as me. How dare she!

Nearly three hours into the race and having seen some 10,000 runners, there had to one. It actually gives me something to focus on instead of the heat. She is running slightly slower than me - but only slightly. I begin the chase. For a brief few minutes we go into darkness under a big underpass as we enter Docklands. The cool temperature is fantastic. As we head for the glow of sunlight in the distance suddenly a runner next to me asks me the time. I’m wearing a watch, he isn’t. It was surreal, as though we both just happen to bump into each other in the middle of London. I politely tell him that it’s ten to one, he thanks me and then jogs on ahead. No worries mate.

After 10 minutes or so I catch the woman wearing my shirt, in spite of the fact she is wearing headphones I throw a joke about being a matching pair which, unfortunately, she doesn’t find very amusing. I realise she’s struggling. I ask her if she’d rather I leave her alone. She sticks her earpiece back in and nods. I quicken my pace a little and leave her behind.

The heat by now is seriously getting unbearable. As I head up West Ferry Road more and more people appear to be walking or running slower than me and I’m not moving that fast. Local residents are handing out sweets and drinks to help people along but I don’t take anything. I concentrate on getting to the 16-mile point and hopefully the sight of my folks.

I don’t have to wait long. They are in fact at about 15 miles on a clearer stretch of road. I see my father first and I ease off. Within the next few yards I come across my wife and mother. There is a bit of shade and they hand me some Lucozade. Within seconds I feel my legs tightening up and I can feel my Achilles aching. I decide to keep moving. Seeing the three of them has lifted my spirits immensely but my body is beginning to complain a bit. Having been a supporter at previous marathons and now having run one, I can’t stress how much having loved ones out on the course counts. Thanks ma and pa. And thanks Rosie, I now know I will make the 26.2 miles...somehow.

I keep telling myself that I’ve run 17 miles before so I can and must keep running before my legs pack up.

As we wind and turn through the streets under Canary Wharf I look up and remember the abseil I did in 2001 off the UK’s tallest building. This is the second biggest challenge I’ve undertaken for charity and I’m not gonna be beaten.

Through Docklands there are only a few mile markers so I lose track of how far I’ve come. I suddenly begin to fill my head with maths. How many miles have I run? How many to go? If I start walking now, how much time am I going to lose? Was the last mile marker 17 miles or 18 miles? I’ve never felt so hot or so tired before. In the distance I see two green and yellow chequered towers with the familiar string of balloons crossing the course. It’s the 19 mile marker. Seven miles to go.

Funnily enough, I don’t actually recognise in my head that this is were we are all going to struggle.

As we head out of the noisy, crowded, lively, happy Canary Wharf area we hit an area with no buildings, no trees, no shade whatsoever. It’s gone one o’clock, temperatures are at their peak and there are more and more runners propped up at the road side. There are St John Ambulance people attending everywhere. I get a few hundred yards on from the 19-mile point and I have to stop running.

I walk a bit. I jog a bit. I don’t hurt particularly anywhere but I have no energy. I find a water bottle by the roadside and it has a small dribble in it. I take a gulp and that’s it. I try to eat some of the dried banana I have in my back pocket and I can chew it but have nothing to swallow with. One tiny slice lasts about 10 minutes. Suddenly seven miles seems a hell of a long way to go.

I decide to walk until I can see the 20-mile marker and then I'll start jogging and see how the legs will do. I can walk OK, I just wish it wasn’t so damn hot. Then it happens ...

"Come on skinny grandad!" shouts from the crowd are aimed at a tall gentleman in a red and white stripy top who must be in his 60s. He jogs past me. On his shirt he has the words Skinny Grandad. I decide there and then, I’m not gonna be beaten by this guy. I begin to jog again and discover my legs have some energy back. I begin running again and I’m keeping up with skinny grandad.

We get to about 20 miles and start heading back towards Tower Bridge. The crowds begin to swell and the atmosphere lifts. Although there are more and more runners wrapped in space blankets propped up in any areas of shade, I don’t feel as though I’m going to join. More and more hands are reaching out from the crowd, some full of jelly babies, some holding fruit, some just reaching out for a touch. The temperature is beginning to drop and more clouds are coming in.

It’s at this point the tops of my thighs begin to seriously ache. I’ve left skinny grandad behind so I slow to a walk. I jog a little, I walk a little. The crowd keep shouting "Come on map man, keep running A to Z". I try to smile and nod to them but the stabbing pains from my legs hurt too much. My ankle is also beginning to hurt. The 21-mile marker is in the distance and I know there is still five miles to go. I try and imagine what that distance is back on my training runs and pretend I’m just leaving home for a short run.

I decide it’s time for drastic measures. If I can’t walk off the pain perhaps if I try a short burst of fast running it might help. I begin to jog just as skinny grandad passes me at a steady pace. No way Jose...it’s all the incentive I need. As we pass under the balloons at 21 miles I begin to sprint.

Now picture the scene. The elite sprinters passed by several hours earlier, even the club runners will be home by now soaking in a warm bath or down the pub. I am surrounded by slow joggers, walking wounded, fancy dressed runners running in sweat. The road looks more like a mass of refuges walking out of the east into the city for food. Then suddenly among this crowd a complete nutter in an A to Z map comes belting through any gap he can find as though he’s in a 400-metre dash not a full blown marathon. I get amazing cheers from the crowd and they push me on. Ok it was only lasting a minute or so. I’d jog for a while then burst into a fast run. Eat my dust skinny grandad! But it worked, my legs stop aching.

Then it happens again, my whole family are together at about 21 and a half miles. They give me a huge cheer, I throw them a sweet I got earlier and don’t want and I weave towards them. They shout at me to keep running. They saw me sprinting and can’t believe it. It’s the final boost I need.

I settle into a reasonable jog and Blackfriars is in sight, only three miles to go. Unfortunately my mind begins to work overtime. I look at my sweat-laden watch and realise I have to complete the last three miles in 20 minutes if I am going to reach my five-hour target. I know I can do 10-minute miles at the beginning, but not after 23 miles of running in crazy temperatures.

Stupidly I begin to panic inside and my legs decide to make their final protest! Not only that but I suddenly see a red and white stripy shirt pass me. It’s skinny grandad and he’s still running. I try and summon up every ounce of strength I have.

We are now on the Embankment and Big Ben is way in the distance. The crowds are huge. the atmosphere is electric and the end is so near. I pace myself behind skinny grandad for the next mile. As we make the very, very gentle climb up towards Big Ben, legs say "no". The clock on Big Ben says ten to three.

I have eight minutes to do the last mile. It ain’t going to happen. As I trot into Birdcage Walk I see skinny grandad disappear into the distance. I walk, I jog, I run, I walk. The 600-metre mark comes into sight, the crowd are screaming at us all to keep going. "Run map man, run... come on you’re almost there."

As I reach the 400-metre mark I can see Buckingham Palace and four months worth of training is flashing before me. I turn in front of the palace and the last corner is in touching distance. The familiar red, yellow and blue lines are painted on the red tarmac of the Mall that I’ve seen on television screens for the past six years and I know I have to run.

As I turn that final corner and I see the finishing line I can see the huge digital clock ticking its way to 5 hours 17 minutes .... as I pass the huge grandstands on the Mall and stamp, and I do mean stamp, on the finishing mat, the huge diamond TV screen on the Mall shows my tired face and the time at 5hrs 18mins. This, less the extra 13 minutes it took me to get to the start line, means I did it in 5 hours and 5 minutes. Close enough.

As I stagger up the Mall, climb the ramp for them to cut off my timing chip from my shoes, lean forward for someone to drop a medal over my head, the realisation hits me. I did it.

I survey the bodies that are strewn all over the place. Some with space blankets on, some sucking under oxygen masks, most as pale as ghosts, I actually feel OK. Sunburnt, tired, aching, sore and totally, totally elated.

Will I be back next year? No thanks. A very memorable and exciting day for both myself and my supportive family and I can now tick that box on my life’s list of challenges.

Thanks skinny grandad!