First they'll dive into the cold, murky waters of London's Docklands, fighting to get to the front in what will resemble a crazed shoal of piranha fish. Then, after a mile, they'll drag themselves on to dry land, shedding their wetsuits for a 25-mile bike ride along the Thames, before returning to Docklands for the small matter of a 6-mile run to the finish line.
For many, it will be three to five hours of torture, but this is Britain's fastest growing sport and a record 11,000 people have entered this weekend's London Triathlon, the largest event of its kind in the world.
"Triathlon has become today's must-do event," says Norman Brook, the head of the British Triathlon Federation, which boasts 10,000 members.
"People are getting bored of gyms, where they're already doing a bit of swimming, and running and cycling on exercise machines. Many think they might as well do it outside."
Mr Brook says the explosive popularity of tri-athlon evokes the spirit of the 1980s jogging craze, when, inspired by track heroes such as Seb Coe and Steve Ovett, millions pounded pavements in tight shorts and headbands.
Brook estimates that 100,000 people will enter one of more than 600 sanctioned triathlon events in the UK this year, with many joining one of 350 clubs around the country. Membership of the federation has risen an average 10 per cent per year over the past decade, with a rise of more than 30 per cent in 2006.
Most of those involved in Britain's booming triathlon scene credit the sport's inclusion in the 2000 Sydney Olympics with bringing the once mysterious endurance event, which began in California in 1974, into millions of sitting rooms all over the world. But it wasn't long ago that triathletes were considered by most to be a rare breed of superhuman.
In 1982, a gutsy 23-year-old American student called Julie Moss inspired a generation of triathletes at Hawaii's legendary Ironman competition, in which racers top off a 2.4- mile swim and 112-mile bike ride with a full marathon. Watched by millions on American TV, she collapsed just yards away from a shock victory, crawling and stumbling across the finish line in the dark.
But you don't have to be a super-fit superhero to take part. "I think we've managed to change the image of the sport," says Brook.
"It's now seen as something that's available to everyone and it doesn't have to be a 10-hour slog like Ironman - there are events to suit all abilities."
Among those taking the plunge this weekend are 15-year-old juniors, a smattering of celebrities, including former Olympic rower James Cracknell and the former Downing Street press secretary Alastair Campbell, and John Starbrook from Staines, who at 76 will be the event's oldest competitor.
There will also be an elite field of world-class athletes preparing for September's world championships in Hamburg and next year's Beijing Olympics.
In a summer season of sporting disasters, triathlon is one sport that the country can shout about. The current world adult, under-23 and junior champions are all British. One man with eye on the Olympic rings as he dives into the docks tomorrow will be the reigning world champion, Tim Don, who, aged 14, became hooked after he joined a triathlon club in Hampton, Middlesex.
"I remember watching the first London triathlon in 1998," says Don, 29, "I'm sure there weren't more than 500 competitors, so to see 11,000 people taking part this year and hundreds of triathlons over-subscribed up and down the country is amazing."
Don hopes to complete the London course in less than one hour and 45 minutes, but for most of the competitors lining up at the Excel centre near Canary Wharf this weekend, breaking records or beating personal bests won't be a priority. Like the London Marathon, the triathlon has become a magnet for those wanting to help charities and last year the event raised more than £1m
'The Independent' is a media partner of the London Triathlon
* Sprint (Saturday)
Swim 750m (0.5 miles)
Cycle 20km (12.4 miles)
Run 5km (3.1 miles)
* Olympic (Sunday)
Swim 1.5 km (0.93 miles)
Cycle 40km (24.8 miles)
Run 10 km (6.2 miles)
How they line up
John Starbrook, 76, retired driver
Target: 'Getting round'
I've done 38 marathons, so I'm doing the triathlon just for a bit of a challenge. I was supposed to do one last May but I got a hernia the week before so I had to pull out. I'm doing the shorter sprint event so I can see what it's like and do the Olympic distance next year. I guarantee I won't come last because I've been doing quite a bit of training - running 40 miles a week, swimming five mornings a week and cycling when I can. I'm a bit apprehensive about the swim, but I played water polo until I was about 40, so I'm used to a bit of rough and tumble. I don't really think about the fact that I'm the oldest doing it. In fact I get a bit embarrassed when people ask my age, so sometimes I lie and say that I'm only 66.
Daniel Hanson, 36, consultant
Target: 2hrs 30mins
A few years ago I had an operation on my knee and at one point didn't think I would run again. It sounds crazy but training for triathlon was a really good way to rehabilitate because you can spread the stress on your body across three sports. Triathlon's also a lot more interesting than just, say, running. You get fitter and having three disciplines is a natural leveller because not many people are good at all three events. My weakness is the swim. I'm what's known as a sinker, and it's tough when you can't see where you're going, the water tastes dreadful and people are kicking you. But the hardest thing is juggling my very busy job with training - I squeeze in five or six days a week, sometimes twice a day - but it's worth it.
Tim Don, 29, World Champion
Target: 1hr 43mins
I remember competing in the first Brighton triathlon years ago. There were no sponsors and, after the race, everyone stood around in this little marquee frantically trying to work out the results with pencil and paper. Now we've got computer timing chips round our ankles and photo finishes. I train about 30 hours a week, including five swims, six bike rides, six runs and two gym sessions, plus a massage and physio session. The rest of the time is just sleeping and eating. But it's worth it just for the incredible buzz that comes with really pushing yourself. I also get to race in some amazing places - I've competed in Cancun, New York, South Africa and Thailand.
Triathlon is also a very friendly sport.Reuse content