Give a man a television and he can watch sport. Give that man a race number and he can be on the television, actually doing sport. The lard will disappear, he'll be talking knowledgeably about carbs and he'll be getting up at 6am to go for a run with the theme tune from Chariots of Fire in his head: a fitter, healthier and possibly more productive human being. At least that's what I'm hoping for as I attempt a triathlon for the first time.

An ever-expanding calendar of public-participation sporting events means that most weekends this summer there's something going on that involves the masses in large-scale physical activity. It's the season to be pestered at work by an earnest colleague asking for cash having completed a marathon, a swimathon or even something more imaginative such as the three peaks (where you "bag" Britain's highest hills in 24-hours).

For the participant there is the combination of new trainers and a warm moral glow. You might even, at events such as the London marathon, get to rub shoulders with your sporting heroes and heroines. But what are the long-term health benefits? And are there any disadvantages to using a sporting event to get fit?

Len Almond, director of the British Heart Foundation National Centre for Physical Activity and Health at Loughborough University, is concerned that some people don't realise what they are taking on.

"While a big event can galvanise a person, especially where charity is involved, few people understand the demands of some events. I heard of somebody who prepared for a marathon in just eight weeks, which could be quite dangerous."

He is keen for as many people as possible to do at least some exercise, given that BHF research suggests that many of us don't do anything like enough; and he stresses that there can be real benefits in getting involved in a major event: "If your training is progressive and reasonable you can build up to an event which gives you a real sense of accomplishment. But other people suffer so much that they think 'never again'. You need to access the correct information about your event.

"Some websites, like those for the Great North Run and the London marathon, provide very good information. The problem is that people don't know how to start, and there are a lot of myths.

"For a running event, you need to prepare properly on leg strength otherwise you can damage your knees. Drinking and eating properly is important - a lot of people drink freezing cold water on a cold day, which is bad for you," he explains.

For Liz Kirk, 31, an ethical trade manager, using the London marathon to get fit taught her a lot, and it was a good motivator to do more: "In the end, discipline and fear are the main advantages of using a big event to get fit. You tell people you are doing it and the fear of not doing it, of not being able to deliver - especially when it is your workmates - helps.

"I really struggled to get there. Before training for the event I went running and swimming but it was definitely a big leap for me.

"I gained a great awareness of the negative side of running - I would go to bed afterwards and hear my legs creaking, and I realised the need for balance in your exercise.

"After the event, I had to be helped over Vauxhall Bridge by an old lady. But I'd definitely recommend it - it gave me the confidence to try something new. Afterwards I took up new sports like squash and tennis ... you think, 'I've done a marathon - I can do anything!'"

For me, even training - never mind the event - has brought lots of changes to my life. Next week, I'm going to attempt the Michelob Ultra London Triathlon, having never done anything like it before. Over the past three months, while I've been working towards my goal, I've realised, first, the amount of spending involved - time and money. So there's the first advantage: I now have neither the funds nor the diary space to pay many visits to the pub, and my midriff has undoubtedly benefited as a result. I've even found myself craving fruit as my body's needs change.

Sport can certainly be expensive: triathlon shops seem to exist in a parallel fiscal universe where you should expect to pay as much as £450 for a pair of handlebars. Even keeping down to essentials can be pricey - a wetsuit is compulsory and costs £200 (or £50 to hire).

One of the fun things about these big events is that the "elite" athletes compete alongside regular folk on the day (or rather ahead of you in the distance). Although there does seem to be a real divide between "newbies", as we are known, and proper triathletes.

You hear horror stories of pushing and ducking at the start line. And you worry: will they laugh at my trunks? Will they stand on my goggles and run away laughing?

James Witts, editor of 220 Triathlon magazine, has some words of reassurance. There haven't, he says, been too many fatalities since he got involved in the sport. "But you can expect the odd person to get out of the water and throw up all over their bike. There can also be problems if it's particularly hot.

"But most people are OK. It's definitely a good idea to use a triathlon to get fit - there's a boom in it at the moment, as there used to be with marathons. Compared with running, which is a purely weight-bearing discipline, there are fewer injuries with a tri. It's a more holistic activity and it's fun - and attainable with three to five hours of training a week."

He believes that even if you are the sedentary type you can realistically achieve a sprint-distance triathlon on six to eight weeks' training (a sprint-distance triathlon is usually 750m swim/20km bike/5km run - I am doing the "Olympic distance" which is double that). Witts's magazine is split between features aimed at first timers and people who will happily resort to public nudity in order to shave seconds off their transition time (getting out of the water and on to your bike).

It's good to know that there are other Corinthian interlopers at this kind of event. Like Laura Earle, 31, a marketing coordinator, who would thoroughly recommend a marathon, even if it doesn't guarantee a glittering sporting career. "The marathon was great, and I'd wholeheartedly advise other people to give it a go - it's a focus for you to get fit and you feel a real sense of achievement. I got an amazing response from colleagues, and the day itself is magic.

"In fact, I vowed that I would always watch the marathon, even if I didn't actually do it. The problem is that the year after I did it, I probably ran less than 26 miles in the whole 12 months.

"And Phil [Earle's husband, who completed the marathon with her] was advised not to run at all after the marathon due to a back problem, and certainly not on concrete for long distances. He had to take up swimming to stay fit."

"This year, though, I decided to sell my scooter and start cycling. I used to swim and I like to run so I'm entering the London triathlon."

She's not alone: the Michelob Ultra London Triathlon has around 10,000 entrants this year. According to the organisers, the triathlon is the UK's fastest-growing sport at 10% growth per annum, and 45% of participants in the London tri are first timers - and of those, nearly half take on the full Olympic distance.

The question is not will they survive it, but how much sport will they do next year?