With many distances to choose from, the triathlon, which combines swimming, cycling and running, has lost its gruelling image and become accessible for people of all ages and abilities.
The sport's impressive debut in the 2000 Olympics in Sydney and the fact that the UK has 15 triathletes in the world's top 50 have helped to raise its profile.
"When the sport started in the early 1980s, it was an extreme endurance challenge," says Norman Brook, head of British Triathlon, the sport's UK governing body.
"Since then, triathlon has evolved into a shorter, faster sport and that has made it much more accessible. It's one of those sports you get hooked on."
Fast growing
British Triathlon says it's now the UK's fastest-growing sport, growing at a rate of 10% a year. It's especially popular among 35- to 50-year-olds.
"In many cases, competitors are moving away from team sports for whatever reason but still want to stay physically fit and have a challenge to work towards," says Norman.
"They can do that with triathlons because they are competing in their age group."
It doesn't have to be expensive. You don't need a £1,000 race bike or a £200 wetsuit to take part. Many competitors use mountain bikes or hybrids, some equipped with shopping baskets. If necessary, you can hire a wetsuit on the day of the event. If not, swimming trunks will do.
Among the more popular entry-level events are the super-sprint, which involves a 400m swim, a 10km (6.5 mile) cycle and a 2.5km (1.5 mile) run. Also popular is the three-man team event, where each member completes one leg of the event.
“The three-man team event is a great way to experience a triathlon by just doing your strongest sport,” says Heather Williams, British Triathlon's performance director.
“Alternatively, training for all three sports is more interesting for the person who just wants to keep fit. It’s really important to mix things up a little to stay motivated for fitness. Triathlon provides that variety.
"Training for three sports is a full body workout, so it’s better for your overall conditioning than doing just one. And by combining all three sports, you're less prone to injury because you’re not always working the same parts of the body.”
An adrenalin rush
Ailsa Weir, 28, from Milton Keynes, who has participated in two triathlons, finds a three-event race more interesting than a bike race or a half-marathon.
"I got into it because of my friends," she says. "We train together, compete together and after the race, we drink together, so it's a great way to socialise."
The hope of improving on his girlfriend's results drew Samuel Wallis, 27, from south London, to the sport. But after several attempts, he hasn't even come close to beating her.
"I get an adrenalin rush from competing," Samuel says. "The swim especially can be a real buzz. I like swimming outdoors. It's much better than a pool of chlorine.
"Despite its hardcore image, triathlon is not as intensive as training for something like a marathon. It's a full body workout. You get better all-round fitness."