When Mr Walker first mentioned running the marathon last June, it sounded like one of those fantastic ideas (especially as I was promised thirty other people were running) and something that as a relatively young and fit sportsman would be a bit of fun to do. Five months later, wheezing for 17 miles along the Basingstoke Canal in driving rain and freezing winds, with cow pat on both shoes and blisters on both feet,  I had an inkling it might not be the walk in the park I had expected.

My training began with mere 3 mile jogs – half an hour runs which most people can manage - and I promise, once you get over the mental barrier of running 45 minutes, then an hour, you begin to believe the big one’s within your range. However, once over the 11-12 mile mark, things become a little tougher again - although I’m proud to think I could run half marathons with relative comfort now.

Any marathon runner will tell you that "you don’t train for marathons by running marathons"; that is, you only ever attempt 20 miles in practice. This plan flummoxed me and my fellow runners as we went into the race with 6 miles of unknown ahead of us, which just made the whole thing that more daunting.

However, despite two weeks of late nights and rum and cokes in New York and Barbados, I felt I had done enough to give me a shot at 3hours 45 minutes (15 minutes slower than my original aim but a steady target nevertheless) and fortunately I was injury-free come race day.

My attitude throughout was pretty haphazard and blasé: I was never going to go in for the expensive stopwatch and heart monitor. I have to admit though, the running bug does get you, and I feel it only fair to admit purchasing some lycra shorts and running leggings, and a belt for my Lucozade gel during the race – probably two steps from pure running geekdom.

And as the race approached I did find myself calorie counting and ensuring my glycogen levels were fully stocked and I was fully hydrated.

With my every waking sentence beginning to sound like something from Runner’s Weekly, it was no surprise to find my old friends were beginning to desert me, whilst sinuous middle aged men in luminous hats and gloves cornered me for chats about ‘split-times’ and ‘the wall’. Thankfully the other three Lord Wandsworth College marathon men had also undergone the personality transplant and were switching a morning coffee for electrolyte formula, so there was strength in numbers.

In some ways it was the fundraising which was the hard part of preparing for the race, but it soon became apparent that people really are interested in your madness and are often more generous than you had ever imagined. With four of us working hard for a charity well known in the school (CRY), the £6000 target eventually became a reality, and another sense of achievement could be felt. The thought of the many young people who have died unexpectedly on the sports field was one of the major motivations for me when the going really got tough up in London.

The race itself was a nerve-jangler for the entire week before, right up until the starting gun (or hooter). An anxious night was spent in London on the eve of the race, with odd stares as my co-runners boarded the train clutching their oversized heart outfits, which were subsequently mistaken for red noses, apples and any number of other amorphous red objects. Despite their jibes about being left out, not once did I envy what they were about to go through. The hot weather that had been forecast duly arrived, and would dog all the competitors through the race.

At the start, I teamed up with another CRY runner (not in costume) who took me off at some pace through the rowdy pubs of the East End, where middle-aged Elvis impersonators crooned from balconies and odd men serenaded with bugles from their back-alley doorways. By the time we reached the Cutty Sark (sadly absent due to renovation) I felt the pace had to be slowed if I was to avoid a humiliating breakdown somewhere near the Dome later on, so I fell away from Andrew, my pacemaker, and went it alone. I had made the cardinal error of not having my name on my vest and so the crowd rarely gave me direct support, so I found myself attaching myself to other Toms and revelling in their reflected support. The crowd, as everyone had said, was amazing though – spectators all the way round the 26-mile course, enjoying the sunshine and the atmosphere and making a real racket.

Over Tower Bridge and the halfway point, and I was feeling fine, although a little tight in the hamstring, as I watched the elite runners striding in the other direction. Here I ran for a couple of miles with a chap called ‘Monkey’ who must be cursing me, as he dropped to walking pace somewhere in the tortuous streets of the Isle of Dogs.

I had been given some advice from a friend who said that halfway point in the marathon is at 19 miles, so I tried to keep it calm and steady to that point, although with people walking all around me I wondered when the dreaded wall was going to hit me. Surely my turn would come at any step.

Past the mental barrier of 19 miles and from then on it really was a case of just putting one foot in front of the other and hoping for the best. I had drunk water or Lucozade at every stop, eaten regular gels and jelly babies, gratefully snatched from generous onlookers and ran through each set of showers on the course, but by 22 miles it was left to just me, my legs and a very tired mind.

23 miles and past the Tower of London, but no time for sightseeing now. Monument. Didn’t my friend say he’d be there? And where were my mum and brother? Under a tunnel and more people expiring, then, thank goodness! A cry goes up from my left – it’s my friends from university, all clenched fists and beaming smiles. I’m good for a spurt but there’s still some way to go. All I can think of is the hill out of Lower Froyle back at home. Two and a half miles, I’ve done this so many times now, but have I got anything left? Then another friend, unexpected on the Embankment. This time I know I’m in sight and I let the adrenalin take over as Big Ben looms large ahead.

The last mile is (thankfully) a smooth downhill and I’m going fine now and even manage a smile as I turn onto the Mall. I see the clock ahead and I know I’ll break 4 hours if I keep going. But a sharp pain in my left arm jolts through me and I fear the worst and start to search for paramedics at the finish line. But it’s only cramp, and I’m under the clock and home to grab my medal and goody bag and to face the inevitable pain of 4 days’ stiffness.

It’s an old cliché, but the pain is only temporary, and I’m definitely still smiling and dead proud of my achievement. I can’t believe that talk of the next one has begun already. I guess everyone really has got the bug after all. Now where can I get a stopwatch?