Simon Usborne: It is the exhaustion that attracts you to the race

I have competed in two Olympic-length triathlons – a 1,500m swim followed by a 42km bike ride and a 10km run – and I can confidently say they were the most gruelling challenges I have ever faced.

For my first triathlon, which took place last year at Windsor, I dived into the Thames with 50 others, thrashed about to gain pole position and then swam 1,500 metres against the current in a river that was swollen with rain. That was for starters. At the finish we climbed out, stripped off our wet suits, jumped on our bikes and pedalled like mad before discarding the bikes and running – or crawling – to the finish.

The attraction is the comprehensive workout you get. Most people do triathlons as a target for training. It involves every muscle you could conceivably want to use.

The real challenge is crossing disciplines. The swim is probably the most daunting stage, I can see why people come a cropper then. If you want to finish in a good time – my best was two hours 17 minutes – you have to get in front of the pack. It's like joining a shoal of piranhas – I got punched and kicked, it was really quite violent with thrashing limbs and frothing water.

Everyone has to wear a wet suit if the water temperature is below 14 degrees, which makes you more buoyant so you can cut through the water with greater ease, but then there is the frantic transition at the end of the swim when you pull off your wet suit and pull on your socks and bike shoes. The experts can do it in about 30 seconds.

Triathlons tend to attract people who are not interested in doing conventional races. I am 27 years old and the oldest competitor I have found myself up against was 78-year-old Richard Starbrook, who took up the sport because he was bored with marathons.

The race is demanding so organisers allow competitors to do it in relay teams, but there is a special joy in doing all the sections yourself and a sense of achievement when you crawl across the finish line. I have never felt more ruined and exhausted than when I finshed the London triathlon last year. I would definitely go back and do it again.