Jonathan Thompson reports on Britain's newest health craze

Already, thousands of pasty urbanites have been persuaded to spurn the treadmills and cappuccino bars of expensive clubs in favour of open-air "beastings", complete with press-ups, shuttle runs and bawling instructors.

The market leader, British Military Fitness (BMF), says business has taken off dramatically in the past year, with a 50 per cent rise in demand and more than 10,000 clients meeting in parks around the country.

Such is the popularity that rival companies are moving into the "boot camps for bankers" and the Army has got in on the act by launching a commercial running shoe.

BMF, which runs hour-long classes in Manchester, Glasgow, Edinburgh, Brighton, Cardiff and Reading, as well as seven London parks, has the unique selling point that its entire roster of fitness instructors are current or former officers in the armed forces.

Kevin Lomas, a former Royal Marine colonel and BMF's operations manager, said: "We get results. We've had loads of success stories - people who within a few months have lost three or four stone. We can push them further than they might push themselves."

Around 60 per cent of BMF's customers are women, and Mr Lomas believes the company's popularity has a lot to do with a backlash against gym culture.

"People come to us because they want to get away from the monotony and boredom of the gym - the same four walls, the same music, the same people admiring themselves in the same mirrors."

Following the success of BMF, other companies have begun offering similar army-style exercise - albeit without the military personnel. One of these is Matt Roberts Boot Camp, launched earlier this year by the eponymous celebrity fitness instructor, which has been taking place in five London parks all summer.

What makes a nice girl want to work out with tough guys like this?

"Hands off hips, number 51!" bellows an ex-marine in combat fatigues as I pant around Clapham Common. I am, unusually, running away from the Windmill pub, where I have spent happier times on this same grass, nursing a pint of Ordinary as the sun sets. This time the Windmill is shut. It is only 10am.

Deciding my fitness level is "average" was my first mistake. We have run as far as the lake when I realise it is not. "A member of the public's definition of fitness might be different from the forces'," says Kevin Lomas cheerfully, but it is too late. Another camouflage-clad monster is shouting, "Get those knees up, 51, you are not special!" "But I am," I want to whine, except I can't breathe.

I am not the only beginner. Paul is 41 and a cyclist. He says going to the gym "makes me yawn". Simi is 35, she runs and does yoga and some kind of hardcore ballet. She doesn't know what her fitness level is. I think she sounds like a pro, but she's not sure.

The running hurts. I come last. I let my team down. But they shout for me anyway. Their "Come on 51!"s hammer in my ears along with my blood, and in a sick way it feels lovely. But I never thought I would be glad to see the press-ups start, or gulp down water full of strangers' spit. Apparently there have been five BMF marriages already. After a particularly inelegant grapple during team exercises, I think I am technically engaged myself.

After an hour, the adrenalin is kicking in. "I have cancelled my gym membership," says Melanie, 35. "It's so much better, exercising outside." Kevin comes over. "I think perhaps you belong in the novice class for now," he says, kindly.

I think perhaps I belong in the pub.

Katy Guest