There is no better, more invigorating way of starting a Sunday morning than getting up in your dressing gown and sitting in front of the telly with a plate of toast, watching thousands of other people taking exercise. That's why the London marathon cheers up the nation so successfully.

It is an extraordinary spectacle. Not so long ago, marathon running was a sport for tiny numbers of oddballs. Now it has become a mass participation event all round the world, nowhere more so than Britain. Some 46,500 people loped, struggled or limped round the London course yesterday. It is the biggest of a bewildering number of marathons and other long races. The 28,000 places for May's Great Manchester Run are all taken, and September's Great North Run, which can accommodate 50,000 people, is full too.

The marathon season runs from February to December and takes in almost every part of the country. There are Loch Ness marathons, Robin Hood marathons, Shakespeare marathons, a "Neolithic" marathon, coastal marathons, mountain marathons, and forest marathons. Yesterday, London wasn't the only one. There was also Britain's smallest marathon, Tresco on the Scilly Isles, with a limit of 125 competitors running laps, because the island is only two miles across.

Below that level there are of course all the half-marathons, the 10k runs, the Race for Life 5ks which attract more and more women each year, and so on. Sometimes it seems half the country is at it. And though the lure of toast on the sofa is strong, even I'm a runner, of a kind. When I turned 50 two years ago, a charity asked me to organise a group of friends of the same age into a "fabulous fifties" team for marathon training. One of us fell off his bike, another tore a knee, another couldn't stick the training, and I had an unhappy encounter with a white van while out running which led to months of complications. In the end only one of the six actually ran the marathon. Still, it shows no one is totally immune from the challenge of that 26-mile act of self-abuse.

The marathon story is uplifting because it symbolises our urge for self-improvement, open to anyone from any background, fit or unfit. But of course it's a minority who run - a small minority in two senses. The wider picture is that we British are getting unfitter and fatter. Obesity is soaring. Just over a decade ago, 16% of women and 13% of men were obese. Bad enough, but by 2005 the figures were much higher: nearly a quarter of women and 22% of men. Among kids, there has been a similar jump. Just under a fifth of boys and girls are now considered obese.

So what else, apart from marathon running might really make a difference on a national scale? To change the country's attitude to sport will take more than a bit of government finger-wagging now. It will need something big.

That exists already, of course. It is the London Olympics of 2012, the subject of national celebration two years ago but already buried as a good news story by an unending stream of grim tales of soaring costs, terrorism fears and fighting about who will pay. When Tessa Jowell announced last month that the projected cost had risen to £9.35bn, four times the first forecast, and that an extra £675m would be coming from the lottery good causes fund, she stirred up a storm that - judging by the fury from arts organisations reported in today's paper - is still growing in intensity.

There are big issues here, of course. The country could very well end up ruing 2012. The Central Council of Physical Recreation, which represents 270 governing bodies of sports of all kinds, put it well. Its chief executive, Tim Lamb, fears that money will be taken away from community sport to fund the Olympics, "robbing Peter to pay Paul".

The government fiercely denies that local sport is going to be hammered. Money is tight, and there are innumerable competing groups anxious about the Olympics effect. In the real world, not everyone can win. Politics is the language of priorities, and all that. Further, we have to be realistic. Every huge project of this kind goes over budget, and the unknowable threats of international terrorism give the London Olympics a special extra problem.

Undoubtedly the fears of arts, cultural and sporting groups about their funding need to be addressed. An Olympics that ran smoothly in London and saw off outside threats - but was paid for by shutting down sporting venues and organisations across Britain - would still be a failure. The other side of the coin is that an Olympics that suffered hostile press comment on its overall cost but inspired millions of overweight young people to learn new skills and get fitter - or even to stop bunking off sports - would be a success. Just think of the savings for the NHS.

The health of the nation really is still up for grabs, and it matters even more than cost overruns in London. As the mother of competitive swimmers, I'm still aghast at the lack of the vital 50m pools in Britain. There are just nine of them, with another seven planned. Swimming is the most popular participatory sport, but similar worries exist in athletics and other sports. Whatever the raw figures, not enough cash has made it to the poorer frontline schools to kickstart a better sporting culture there.

Yet it simply isn't true that nothing has improved. Tony Blair's pledge seven years ago that every pupil would have the right to two hours of sport a week has not been met. But 80% of pupils have got there, up considerably on recent years. Since the turn of the century, 1,114 sporting facilities of different kinds have been completed across the UK. Of the Olympics money, £340m is pledged for community sports programmes and funding for clubs. When the games are over, a lot of world-class venues will be left behind. In Spain, participation in sport has doubled since the Barcelona Olympics of 1992.

So there is plenty of reason for optimism, and certainly no need yet to throw up our hands with our strange national mix of glee and despair at another looming failure. What we need from ministers is the political guts not to see the Olympics only through the prism of national prestige, but to think always about the millions whose lives could be transformed. Our local park teemed with even more runners than usual yesterday, some of them no doubt inspired by the marathon. The Olympics will present an even greater opportunity to transform the lifestyle of the nation.

· jackie.ashley@theguardian.com