It's because I think I could. In fact, screw it, I know I could. I'm pretty sure I could do one tomorrow, actually, as long as by "do one" you meant shamble round the course at slightly faster than walking speed for just over five hours, stopping perhaps to vomit occasionally and asking those guys on the camera motorbikes for a lift.

But I'm not going to do that. No. Because that would be a half-arsed way of doing it, and there's little point in building yourself up psychologically for something like running a marathon, climbing a mountain or setting fire to your own house to claim it on the insurance, and then just scraping through it. If I'm going to commit arson and insurance fraud, metaphorically speaking, I damn well want the place razed to the ground and to stumble, blinking and coughing, out of the rubble with at least a minor (but convincing) case of smoke inhalation.

Here's the maths: one marathon = 26.2 miles / 42.2 km - call it 42 km for simplicity's sake. 42 km is the equivalent of:
a) 10 km/h for 4 hours 12 minutes
b) 10.5 km/h for 4 hours
c) 12 km/h for 3 hours 30 minutes
d) 14 km/h for 3 hours
and I reckon option (c) ought to be achievable. I'm already running at 14 km/h for 45 minutes without much problem - apart from the DAMN TEDIUM OF IT ALL, THAT IS... ahem - sorry. I need to get that up to at least an hour at the same kind of speed, three times a week. Then try for longer. And longer. The problem is that treadmills don't work so well for distance targets - you picks yer speed, and off you go. So I'll have to build up a ton of hourage, rather than mileage.

And that's it: three and a half hours of running at 12 km/h in, say, six month's time. Three and a half hours. One quarter of a flight from Tokyo to London. The period of time from arriving at the office to wondering where to go for lunch. Seven episodes of Blackadder or Fawlty Towers. Doesn't sound so bad when you put it like that, does it?

The only problem, apart from blisters, shin splints, ingrowing toenails, muscle cramps, inflamed tendons, dehydration, dizzyness and possible heart failure, the *only* problem apart from all that is the boredom of all the training. It's not much of a hobby, when you think about it. Running. Running for a bus is one thing - you have the thought, "oh shit oh shit oh shit shit shit I'm gonna miss the bus oh shit..." at the forefront of your mind, with either a "phew!" or a "crap!" appended, depending on how things turn out. It's hardly the dreaded wall, now, is it? But running for three or four hours against the better judgement of every sinew in your body? It's different. Difficult. A mental challenge. Which is, as I think I'm beginning to realise, why I'm going to do it.

I don't want to do this because I want another hobby; I'm not planning to do one because I want to end up the kind of person who runs marathons for fun, who flirts with the idea of moving on to triathlons and ultra-distance running, or who sits around discussing his personal best with other runners. Not at all. I want to be the kind of person who ran a marathon once because he wanted to see how hard it was, because he wanted the t-shirt - even if it was only so he could hide it at the back of his wardrobe. I want to do it precisely because I want to *prove* that I can do it.

Which is why I'm only going to do it once. Train for the next six months, do the damn thing, then stop. Never go near a treadmill again as long as I live. Stick to real-world cycling and the odd multi-gym contraption for exercise. Start fencing again. Develop other hare-brained schemes. Not boast about having run a marathon. Do other things. Prove other things.

And occasionally, just occasionally, I'll get the secret t-shirt out from the back of the wardrobe and wear it... but only when no-one else is around.

What's your secret t-shirt going to say?