Simon Usborne: Triathlon hurts. But my pain felt good

It's strange, when you look at the leg-buckling exhaustion that strikes triathletes when they cross the line, to consider that it is one of the fastest-growing amateur sports in the country. There are more than 350 official triathlon clubs in the UK, and the biggest event in the calendar, the London Triathlon, attracts 13,000 masochists.

I raced in two Olympic-length triathlons four years ago, and can say with no exaggeration that they were among the most dispiriting, painful and unpleasant experiences of my life.

Like Jonny Brownlee, I collapsed on the floor afterwards, a sweaty heap of limbs that would ache for days.

So why did I do them? Why does anyone do triathlons? Because in spite of the agony – arguably because of the agony – they are deeply rewarding.

My second triathlon, in London, began with what for most competitors is the most daunting discipline: the 1,500m (one mile) swim.

I jostled for space among 400 men in wetsuits before a klaxon unleashed what resembled a shoal of cannibalistic piranhas. We thrashed and elbowed each other while trying to breathe and settle into a stroke. Twenty-seven minutes later I left the water.

The bike ride, at 42km is short enough to go all-out (I managed it in 66 minutes) but I paid for it after the second transition, when my thighs really weren't keen to run 10km. By the halfway point I was desperate to stop, my stomach cramping and my right knee beginning to seize up. But you can't quit a triathlon, so I pushed through and gained speed in the final quarter to the finish line.

I'd completed the race in two hours and 21 minutes, more than half an hour slower than Alistair Brownlee's winning performance yesterday.

I did it simply because pushing yourself can feel nice. Unlike the pros, however, I haven't felt the need to do it again since.