What, after all, could be so bloody hard about a triathlon? If, like me, you had dined out for two whole years on completing the marathon, 26.2 grinding miles of it, then the Olympian triathlon distance doesn't seem so impossible. A 1,500m swim - I can do that. A 40km bike ride - I did longer when I rode the London-Brighton 20 years ago, and you can always stop pedalling for a few seconds without stopping, something you can't easily do when running or swimming. A 10km run - I do that every Saturday. Sign me up, Body&Soul, I'm in!

Of course, I'm an idiot. There are just over two months to go to the grand Mazda London Triathlon, to be held in the Docklands on August 9 and 10, and things are not looking brilliant. Well, how could they be? Why had I chosen to forget that the last time I rode a bike ended with me rolling across two lanes of Islington traffic, my front wheel bent over like a pipe cleaner and my confidence even more buckled than that? Or that the only way I knew how to swim was by a dogged breaststroke, head out of the water like a labrador? So this first part of my journey concerns what a complex and challenging thing a triathlon really is, and why I should - this time - have listened to my wife.

We're good at triathlons in Britain

First, some history. The triathlon is younger than me. It was invented in the mid-1970s, as one might imagine, by sun-kissed Californians, who were bored with surfing and hadn't yet discovered work. It took 26 years for the triathlon to make its debut as an Olympic event, in Sydney in 2000. And, for some reason, we in Britain - rubbish at so much else - are good at triathlon and getting keener. Seven years ago 1,000 competitors participated in the London Docklands event; this year there will be 13,000 of us. No wonder triathlon is the preferred sport of City folk.

Not that the triathletes I met on a hot weekend in early May in Eastbourne, East Sussex, were City types. Herbalife, the nutrition sponsors of the London Triathlon, each year arranges for a group of promising triathletes to be given expert coaching. Someone humorous thought it would be a good idea if I joined them.

The weekend was to be spent in aspects of all three disciplines, plus the arcane business of “transitioning”, the art of getting from being a swimmer to being a cyclist, and then stopping being a cyclist and mutating into a runner. There was a student, Ryan, from the Wirral, Matt (40 next birthday), and 28-year-old Nick, all of them normal guys made abnormal by their desire to excel at this sport. They are lovely blokes, being trained by the equally adorable Bill Black, probably Britain's most successful triathlon coach.

I had driven down with my new unridden bike in the back of the car, and the first thing we had to do was run hilly circuits of the Downs. They all sprinted off and did three laps averaging ten minutes; I did two averaging 17. Bill was encouraging. In the afternoon I tried to get into a wetsuit for the second time in my life, eventually needing help to tug the thing up over my well-developed stomach muscles and fasten the Velcro at the back. I had never understood the meaning of the term “uptight” until that moment. Bill was patient.

I breaststroked sedately in my own lane while the contenders were taught how to cope in a turbulent lakeful of ferocious frontcrawlers. Duncan Rolley, our swimming coach and former British champion, decided to take me in hand. “Afraid of putting your face in the water?” I agreed. As he pointed out, the problem is that the more the head comes up, the more the legs and body go down, the slower you get and the more effort it takes. “Try putting your face in and breathing out every third stroke, then build it up from there.” I did. Bill was happy.

The next day it was a 40km hill race for them on their bikes, and a painful parading of the sea-front cycle path for me, the saddle doing significant pudendal damage as I fiddled with the gears and completely forgot how absurd I looked in a breast-hugging tri-suit. At no stage did I feel in command, failing twice even to get my right foot into the clips on the pedal.

By the time I arrived home I had done a fraction of what the lads had done and I was exhausted. With Bill's encouragement I felt I could make it, but that it would be harder than I'd realised. And most of the hard part would be in the mind.

Well, in the weeks since Eastbourne one thing has gone well, one OK, and one badly. I have started on interval training in running. Basically I sprint 200m, then walk 100m and repeat four times, followed by a 400m sprint with a 200m walk, repeated twice - and it works. Late last month I ran the BUPA 10km in London in 56.34, which was roughly what I wanted and five minutes better than when I'd last competed at that distance.

Using Duncan's build-up method, my swimming is picking up slightly. I manage every second length with my face in that hideously wet water, and I can complete the 1,500m in 46 minutes, compared with the floating-corpse 55 minutes I started with.

I was sure I'd be killed by a night bus

The bad news is that I am still terrified of getting on the bike. I tried it once early one Sunday, before even the birds were up to laugh at me, and spent 40 minutes being sure I'd be killed by a night bus. This is a problem because tomorrow I will be hopping on my bicycle to do the London to Brighton bike ride - all 87km (54 miles) of it - and at this rate it will be a humiliation. But that's what you like reading about, right?

So, David's triathlon: three indisciplines. Too old. Too fat. Too scared. But I'm still going to do it, buddy.

There are still charity places and team

places to compete in the Mazda London Triathlon. For more information visit thelondontriathlon.co.uk

To join his group on The Times Health Club visit http://triathlon.groups.timeshealth.co.uk.