It is time. Old enmities have been resolved, childish things cast aside and my affairs put in order. I've got someone in to look after the cats and I've told my sister that if I had any insurance or pension policies, she would be the named beneficiary on them in the event of my death. It is time, in short, for my first run.

I'm supposed to work in five-minute bursts; an apt term, given the strain my fat-lagged heart will be under. The plan is to run for one minute, walk for four, and gradually improve the ratio as my fitness builds. Do you know how long I manage? Thirty-seven seconds. I check the stopwatch for signs of tampering and myself for signs of emphysema, leg amputation, inadvertent donning of wellingtons filled with molasses before leaving the house - anything to explain the stunning inadequacy of this figure.

I walk for four minutes and 23 seconds, trying not to recall that people can run a mile in substantially less. Other runners - perhaps I should just say runners - zip past. How are they doing it? Are they experiencing a lesser form of gravity? Or have they just not spent the last decade cramming Mars bars down their gullets with one hand and channel-hopping with the remote in the other?

My second bout lasts for 90 seconds. This is almost three times as long as the first, but - and here's the thing - it's still only 90 seconds. Add to this the fact that I am now puce, sobbing, on my knees, and grabbing at passing children in the hope of stealing their bikes and freewheeling home and the situation becomes even less edifying.

And, it turns out, I have peaked. I manage two more (separate, I feel I need hardly add) minutes before staggering through the front door and falling on to the sofa. (Note to self: move sofa closer to door.)

As I lie there with my vestigial muscles screaming with outrage and the blood pounding through my barely dilated vessels, I realise that I have achieved something rare - something possibly unique: superlative mediocrity.

Next week: Tim Dowling swims in an endless pool