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So on 22nd April 2012, at 9.45am, I found myself in a crowd of over 30,000 people, edging towards the official "mass" start line - where a sharp chorus of bleeps signalled the start of the suffering for the steady surge of runners that I would have to find my way through to achieve my target time. As I was pressed over the line, I realised the next 3 hours running were the product of months of sacrifice. 

From the simple sacrifices - less Big Macs and more bananas - to the lifestyle changes I had to make - tee-total for 6 weeks - the Marathon was dictating much more than just the necessity for tonnes of tedious training runs. But, as I soon came to learn, just reaching the start line in Greenwich would be as testing as completing the course that follows. 

It being difficult to put into words, some stats should put this into perspective. Since January I have: Lost 1 stone; Run 520 miles - over 100 hours; Eaten 28.2 "extra" days' allowance of food; and Burned 69,414 calories running.

So crossing that start line wasn't the start of a journey, it was the half-way point. And working my way through the masses, I felt at my peak - light and trim, with a comfortable stride and what felt like boundless energy after a week of carb loading. How wrong I was..

I made two mistakes early on. I spent the first 5 miles weaving through slower runners - increasing my overall race distance by over a mile and wasting precious energy. I also started marginally too fast. I was running very consistent splits, but they were projected to lead me in for an amazing time of 2.42.00. I didn't realise the extent of the damage this fast start would cause until I caught up, and eventually passed, Mr Fraser - my running coach who had started a minute before me, and who was garunteed to run a solid sub-2.55.00 run.

My fast splits began to lag after I crossed the halfway (13 mile) point that was the magnificent Tower Bridge. Anticipating "hitting the wall" early, I slowed myself down to the pace of the runners around me. This worked until mile 16, when Mr Fraser breezed back past me, along with half the people I had worked so hard to overtake. 

From mile 18 I knew finishing the race should be my aim - not a time. My legs had dropped off a cliff, I could feel I was running on empty, and my muscles were threatening to cramp up. I got my head down and battled through the Canary Wharf area - the crowd being particularly helpful at this stage.

By mile 20, my legs really, really wanted to stop. They declared this very suddenly through a gun-shot-like pain in my right hamstring. I jumped up in the air in shock and felt a fist-like portion of my muscle spasming out of my leg. This was the low point of the race for me. Stretching out the most horrific cramp I had ever had, when 6 miles separated me from the sacred Oxfam massage, and while a man dressed as batman hobbled past me to the cheers of the crowd. 

I honestly cannot remember much more of the race after I got myself going after the first cramp. I know i had to stop with more cramp twice more, and these two stops ultimately cos me my sub-3.00.00 time. But by then my mind was so fried from the heat, but most of all from the agony, that I couldn't conceive anything but the runners' feet in front of me. I passed friends and relatives but didn't register their cheers, and passed Big Ben, the Houses of Parliament, Buckingham Palace and The Mall, without relishing the unique experience it was to run past them all with thousands cheering me on. 

Emotions when I crossed the line. You'd expect me to say it was a rush of happiness, a huge weight off my shoulders. Honestly, I was emotionless. I was that tired. Autopilot was engaged, and the empty vessel that was by limp, broken body somehow carried itself to a piece of grass where it promptly collapsed. 

It was only after 30 minutes or so that the concept of emotions returned to me. I was delighted that the ordeal was over. Disappointed that cramp sabotaged my time. Upset that my legs had changed into a mixture of rock and jelly. But happy and proud that I didn't give up, that I had the grit and determination to fight through "the wall" and subsequent cramp set-backs to stagger a pretty long distance to the line. That, after everything, was my proudest achievement. When everything from head to toe is screaming at you to stop, even for a few minutes, but you carry on plodding - that is a real achievement that I'll always be proud of. 

Days after I can conclude that, at the end of the day, I'm 18, inexperienced and I should have many more marathons in me. I'll give London another crack of the whip one day, and I know I'll have it in me to beat it once again, hopefully in a more impressive time. Still, 3.06.00 is still a rather good time, one that plenty of club athletes would be proud of - so I will tolerate it.. for now.

I'm genuinely thrilled and moved by all the donations I have received, and after chatting with the Oxfam team a few times, I'm humbled to be handing over that money to such a hard-working and happy charity.

Signing off, proud and thankful, Alex King.

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