This coming Sunday morning, Dan Jotcham will be in the cold and mucky water of London's Docklands, waiting for the start of the world's largest triathlon. He has trained five times a week for six months for this competition, got his tetanus jabs up to date, and will have just drunk some flat Diet Coke which, according to triathlete lore, will kill any waterborne nasties he swallows. At 7am, Jotcham will hang back at the start line, ignoring the tussle for a prime start spot in order to save energy. Then the gun will fire, and the water around him will erupt into a churning maelstrom as thousands of amateur athletes barge into, wriggle over, scratch and kick each other, to get ahead.

In 2004, the London Triathlon attracted 4,800 participants. In 2006, the figure was 9,000. This year, Jotcham, a commercial finance manager, was one of more than 12,000 people who applied for the 11,000 available slots. The British Triathlon Federation saw its membership soar by 32% last year, and says that more than 100,000 people competed in one of its 400 sanctioned events.

For spectators, this surge in popularity for triathlon can seem puzzling: it is not an easy sport. There are various standard distances, from the super-sprints at 400m swim, 10km bike ride and 2.5km run, to the Ironman, which starts with a horrifying 3.8km swim, continues with 180km on the bike, and ends with a marathon. Sunday's main races will cover the Olympic distance - with the winners finishing the 1,500m swim, 40km bike ride and 10km run in around one hour 45 minutes for the men, and one hour 55 minutes for the women. Very fit amateurs, the kind who would hope for three hours 45 minutes in a marathon, will be aiming for something like two hours 40 minutes.

Many triathlon competitors have run marathons before. Charlie Wilson, a research scientist based in Oxford, ran the London marathon last year, but found "the training was too repetitive". Triathlon provides a similar challenge without the monotony, and gives equivalent health benefits. "Without something to aim for athletically," Wilson explains, "I tend to turn into a pie monster without really trying."

Pie-bellies don't stand a chance against triathlon training. Every week Jotcham swims 2km, cycles up to 100km, and runs 30km. "The sheer amount of aerobic exercise is good for aerobic conditioning and weight loss," says Richard Hobson, a former Great Britain triathlete, and now an elite-level coach. But triathlon comes into its own because of its variety. "If you just do one discipline, you stress specific parts of your body," he says. Runners get joint problems, for instance, whereas swimmers are prone to shoulder-trouble. Triathletes get a more balanced physique, with less specific stress.

For Sophie Hector, who works in advertising, training for a triathlon was a way of introducing specific targets into her exercise regime. Without them, she says, "I find I coast, just take my foot off the pedal and don't bother." Hector used a personal trainer to guide her through a 10km run last year, and then to step up to triathlon training. She currently trains four times a week, and regularly competes in sprint events.

For those without a trainer to put them through their paces, the prospect of such full-on preparation may seem a lonely prospect. But, says Hobson, there is support out there - particularly if you train with a club. "Perhaps because almost everyone coming into triathlon is doing something new, they start with an open mindset. People helping other people breeds a great atmosphere. It's a very sociable sport, and you can compete on many different levels."

And how much work will it take to reach competition standard? Hobson says that a non-sportsperson should be ready to complete the Olympic distance after doing two 45- to 60-minute sessions of each of the three disciplines every week for 12 weeks. If you are already pretty fit, that kind of training should see you home in a decent time.

Most competitors are weaker at one of the disciplines. Jane McMenemy, a lawyer from Clapham, is an experienced swimmer. "If I'm swimming I know what it feels like to go through the pain and really push myself. If my legs hurt when I'm running, or I feel tired, I kind of think everything's gone wrong and I just want to stop." Again, training with other triathletes can help, because you will learn from each other's strengths and experiences.

Jotcham says he is "intimidated by the Borg-like machines of men who cycle uphill faster than I can cycle downhill".

"A good bike is important," says Hobson, "but what is absolutely essential is that it fits you. Make sure you get it fitted by someone who knows what they are doing."

For most amateur triathletes, swimming presents the greatest challenge. McMenemy has witnessed many seasoned runners ploughing up and down the pool with poor technique. "They are so used to doing exercise where your heart rate rockets, and you sweat. Drilling slowly in a pool is frustrating and doesn't feel like 'proper exercise'." But without decent technique your pool times won't improve - and you could leave yourself exhausted before you even hit shore. Experienced triathlete Neil Appleton recommends learning how to "balance on your chest, so that you can slip through the water without having to kick particularly hard".

The transition sections between disciplines are also crucial to get right. It's much easier to shave a minute off transition times than off a 10km run. Know precisely where your bike is, and how to find it at changeover and learn how to deal effectively with shoes and clothing. "Practise training through transitions," advises Appleton. It is important not to relax, he says. "The next phase uses different muscles, so resting in transition is a waste of time. It won't feel like it, but it's true."

Having so much to think about is part of the appeal for many competitors, says Hobson. "It's like with the kit; people love gadgets." But kitting yourself out for a triathlon doesn't come cheap. A decent bike will cost anything from £500 upwards. You will also need a wetsuit, tri-suit (which goes under the wetsuit for cycling and running) and helmet. You can buy a starter kit with the whole lot for around £1,000, but check out the government's Cycle to Work scheme, which allows you to buy your bike out of pre-tax earnings. Either way, triathlon is certainly not for faddy types.

· thelondontriathlon.com; tritalk.co.uk; britishtriathlon.org