As any contented parent knows, it is best never to look closely inside your child's school bag. In this way one avoids all kinds of unwanted surprises: invitations to musical concerts, good work assemblies and parents' evenings, as well as the usual flick knives, chewing tobacco and so on. But, on a wet Friday in June, there is Maddy (eight) home from school, bag open, waving a piece of paper under my nose.
"Daddy, Daddy. Let's do a triathlon! Can we? Can we? Oh please!"
Naturally, I am about to snatch the paper from her hand and burn it but Caitlin (22) and Niall (15) – fatally present – make an interception.
"It's at Castle Howard, Dad, only down the road. Everyone's doing triathlons now!"
Reluctantly, I take a look. There are races to suit all ages and talents, plus photographs of families apparently enjoying themselves. Triathlons have clearly moved on since the days when only psychopathic masochists did them. The Castle series of events (at three English castles and a French chateau in 2011) also promises athletic fun in superior locations. This one starts with a swim in His Lordship's lake, moving on to a bike ride through the ancestral acres followed by a short run around the stately pile.
"You can easily do the cycling," offers Caitlin. "You'll have residual fitness from the Etape du Tour last year."
Residual fitness sounds like some nasty sludge at the bottom of my drink bottle. Not only has my wonderful Giant Advanced Defy road bike barely moved since its successful ascent of the Col du Tourmalet in the Pyrenees, but I have a bad back and am making regular visits to the osteopath.
A week later, nevertheless, we are all in training. Another week later and Caitlin asks: "What did we talk about before the triathlon?" There is a silence at the dinner table: no one can remember. I love that phenomenon – the way something can take over your entire life for a few weeks, then disappear. My backache has gone and I am fully engaged. I buy Triathlete magazine and even contact a triathlon coach, Liz Scott at The Trilife, to seek out the best advice. Liz tells me that "transition" is crucial. I have to practise moving from swim to bike, and bike to run, especially the latter. We manage a swim in the river Ouse, but there is so much rainwater coming down that we get swept downstream and have to give up. None of us feels like leaping on a bike afterwards. Maddy has a hot chocolate, a chapter of The Hobbit, then falls asleep.
On the Saturday before the race we load up with camping gear and head out to Castle Howard, 15 miles north-east of York, and a fine example of how to turn wads of 18th-century cash into a superb location for both television drama – in this case the 2008 film of Brideshead Revisited – and a triathlon. Within its 13,000 acres are obelisks, temples, woods and lakes galore, easily sufficient to mark out a 400m swim, a 23km bicycle route and a 4km run – the tasks set for myself and the 225 other competitors in the "Sprint" category.
Caitlin is doing the same, while Maddy and Niall are tackling shorter distances a few hours after us. Earlier in the day will be the Olympic distance category in which contenders for the British 2012 team, including Mark Threlfall and David Bishop, compete alongside participants of all ability levels – a rare occurrence in modern sport.
At 9am, Caitlin and I are down at the boathouse. She's looking like a frightened rabbit, and I'm sure I am too. Neither of us is sure we can finish. Fortunately there are other first-timers there, happy to poke fun at themselves and their limited chances of survival. The triathlon's explosion in popularity in Britain follows the pattern set in the US, where it has become the leading "adult entry" sport of the past few years.
"Of course, I've trained hard," says one man from Kent, with sarcasm almost as heavy as his middle-aged spread. "Do you think a body like this comes easy?"
Not everyone, however, is a novice. Earlier, as we prepared our bikes in the transition zone, we noted that some competitors had laid out their kit in perfect order on a towel. Others had neat arrays of power foods, tiny spaceman sachets of edible rocket fuel.
I tuck my slab of Grandma's fruitcake out of sight inside my shiny new running shoes. Just seeing the shoes sitting there is a constant reminder that I have done no training whatsoever for the run – any training, I had figured, would only exacerbate my dormant back injury. Now, that decision feels a little rash.
Back at the lake we are briefed by Brian Adcock, the race director, and then enter the water. It's not too cold. Mud is warm. Dig in toes. Is this the start line? Should I be at the front? Sorry I can't hear with my goggles on. Klaxon. Go! Face down. It's totally black down there. I wouldn't mess with my toenails, mate. Ouch! Oh God, a creature! No, only weed. Look up. Oh, it's that way, is it? Wetsuit is so buoyant – feel like a giant water boatman insect, skittering sideways. Around the red buoy. Turn for home. Don't think I can keep this front crawl going much longer.
Crawl was discovered by Europeans when a Native American swimmer named Flying Gull was invited to London in 1844 to race some Victorian gentlemen in one of the city's newfangled swimming pools. The American thrashed the long pants off the Englishmen, who condemned his novel speedy stroke as "ungentlemanly" and "barbaric". Ha! Now I'm panting and only capable of gentlemanly breaststroke and everyone is going past me. Bloody barbarians.
Finally it is over. I jog up the grassy hill to the transition area. This is now a scene of desperate activity, but I refuse to be hurried, taking energy from my secret weapons: fruitcake and CherryActive juice. Niall and Maddy are screaming at me from behind a barrier, "Get on with it, Dad! Faster!"
Eventually I get on the bike and start the ride: a superb circuit taking in views of the North York Moors and Ryedale. There are a few nasty, short climbs, but several long descents too. I overtake a woman on a hefty mountain bike, its back rack laden with energy bars. I slow down a little, only too aware that I have the run to come.
Back in transition, I rip off my bike shoes and jam on my running shoes. Damn. Grandma's fruitcake. I remove the shoes and scrape out the fruitcake.
"Couldn't have asked for a better day, could we?" says a man next to me, apparently in no hurry. He is right: the early cloud has burned off, leaving a lovely warm, sunny day. I tie my laces and throw off my cycle shirt. Ready.
"We drove up from Essex yesterday and…"
But I am gone.
The first half mile is excruciating. I get stomach ache and feel horrible. Fortunately the course is downhill, then flat. I stop and massage my stomach then set off again. After a long, slow hill I pass the Temple of the Four Winds on my left. Designed by Sir John Vanbrugh in the 1720s, it was an elegant refreshment room for the aristocracy. For us, however, there is just Brian Adcock's mum handing out cups of water at the bridge below – the sense of this being a family occasion is perfectly genuine. We then turn and head up past a series of lakes and the house itself, before a final push up into Ray Wood and back to the finish in front of the main house.
I don't feel too bad, really. Race adrenaline is still zinging through my veins. I've come a creditable 69th, out of 250-ish. Caitlin finishes too – a sterling performance for someone who until a month ago had not been near a swimming pool for two years, and had never been on a road bike.
The real test, however, is to come. I am much more nervous about Niall and Maddy than myself. If either gives up, will that spell the end of all sporting ambitions for them? I reckon Niall has the toughest job; I know I would have struggled to expose myself to the possibility of such a public failure at 15 – and he has done minimal preparation. He starts before Maddy and the swim goes well. But then in the bike section – he's on an unfamiliar high-spec mountain bike – he crashes twice. Nevertheless, he gets back on, both times, and makes up enough time on the run to finish well.
Now for Maddy. You'd think the eight-to-10-year-olds would be impervious to pressure. You'd be right. It's the parents who are cracking. While Brian warms up the youngsters and has them laughing and joking, the parents are swooning with the tension. One man is dragged away by his wife, "before he gives himself a coronary…" The kids, however, are perfectly all right, and the race proves to be a joyful celebration of energy and youth. At the start, Maddy looks very small and childlike, but by the end she appears to have grown several inches, and comes striding down the path from Ray Wood to the finish miraculously transformed: she looks like an athlete.
We end the day lazing on the lawns of Castle Howard, chattering excitedly and watching the medal ceremonies to which, one day, some of us may aspire. Later I chat to Brian, who tells me that 700 competitors attended, and that "some families are even doing the whole Castle series of five triathlons throughout the summer".
I'm going to think about that. There's one in France that does look rather inviting, and Paris is within striking distance.
That night, putting Maddy to bed, I lift down The Hobbit for our usual chapter, but she pushes it away.
"I want Triathlon magazine," she says. "Is there a section for eight-to-10-year-olds?"
