EVEN IN my salad days, I was more of a meat and mashed potato type: more beer than isotonic drinks. At the very height of my teenage sports career, I used to hide behind the trees that lined the sports ground while the football squad sprinted around the pitch, and only joined in the training session when it got to the fun bits: five-a-side and so on.

So why do my thoughts turn to competing in the London Triathlon, a gruelling test of endurance in the water, on bicycle and on foot that makes a marathon look like an option for wimps? Why would anyone want to spend an hour splashing about in the cold and revolting water of London's Docklands, followed by an hour-and-a-half on a bicycle, capped by a hearty 10-kilometre run, which could take me another hour, or depending on my condition by this stage, much longer?

Before anybody else provides the answer, I'll say it first: I'm a classic case of a sad, early-middle-aged man trying to prove that my physical powers are not in irreversible decline. As to where this compulsion originates, many people would put it down to vanity.

I'm not sure I vividly remember my father's first heart attack when I was about the age my sons are now, and since he died before reaching 50 I know that I can make no assumptions about good health and long life. But it has taken me a long time (and a couple of marathons) to raise my target as high as the triathlon, in which I will make my debut classified as a veteran.

My first tentative attempts at sport for fitness, rather than for fun, came when my first son was born 10 years ago. Until then, I had only ever sprinted on land, and my idea of swimming was to do a length as fast as possible and then take a 10-minute breather, preferably in the sun.

Soon I made an important discovery: that it is quite easy to swim for a long time so long as you do it slowly - a rule of thumb (or should it be rule of limb) that applies equally to running. Years later, I knew I had cracked long-distance running when, 15 miles into my first marathon, I was overtaken along a straight by a couple of spectators strolling along with a baby in a pushchair.

My inspiration in the water was Dawn Fraser, arguably the greatest female athlete of the century - and I will argue in favour, since she is my only brush with sporting greatness - whom I used to see at the pool I frequented in Sydney. Fraser won the 100 metres Olympic freestyle three games running, and would have won a fourth if she had not been sacked from the Australian team on petty grounds. Her precise crime escapes me, but it was in the order of talking after lights out in the Olympic village. After a 20-year retirement, most of it spent at the bar of the pub she owned, Fraser decided to compete once more in veterans' competitions, which was why she was ploughing up and down the same Sydney pool as me.

The years and the lifestyle had transformed the strapping young athlete into a statuesque middle-aged woman, but still she burned off relays of super-fit 20-year-old men from the university swimming team who trained alongside her.

Most amazingly, she seemed to swim in slow motion, one long majestic stroke to every four of the young men's, and still she sped ahead of them, eating up lengths as if they were widths. Needless to say, she won her age group event at the Veterans' Olympics in a time that would have brought her gold against competitors 10 years younger.

I decided that I, like Fraser, would swim in slow motion. Predictably, and unlike Fraser, this had the result that I moved extremely slowly through the water. But I found I could keep going pretty well indefinitely.

People often assume that, in addition to the physical exhaustion, swimming, running or cycling for a long time must be mind-numbingly boring. I can confirm that, to start with, it certainly is. But soon the very repetitiveness of the physical task allows your mind to slip into neutral, and then you find your train of thought heading off in all sorts of directions.

This sort of active meditation is, I suppose, the principle behind all of those Eastern mind and body exercises from yoga to t'ai chi. At any other waking hour, I'm far too busy to get round to thinking.

As for the idea that time spent with the children is quality time, it might be for the boys, but for me it is a cross between stand-up comedy, lion-taming and sitting in judgment at the Central Criminal Court. Training is my quality time, when alone with my own thoughts. So next time someone asks me why on earth I want to put myself through the triathlon, I'll say I'm just looking for some peace and quiet.