How do you score?

Remember the torrential downpours last week? Well spare a thought for a group of sodden Londoners, enthusiastically taking the Government's advice to put a little exercise back into our lives.

I recall the bad weather with such clarity. The session started timidly under a blanket of light cloud, then suddenly it turned cold and the rain was sheeting into our faces at 45 degrees. But that doesn't stop a young man in battle fatigues and army boots barking: "Get those knees up higher!"

I fully expect him to conclude his instructions with "... you 'orrible little man" but, instead, he orders us to do three more sets of abdominal crunches and 20 press-ups before we trudge off on another half-circuit of Battersea Park.

"Good running, but is it hurting?" screams our battle-hardy instructor. Breathless and exhausted, I can't reply. No one is more surprised than me, however, that this is the most feel-good experience I've had in ages.

There are a dozen of us in the group this evening and the mix between men and women is fairly equal, the average age around 30.

The common ground is that all of us have signed up with British Military Fitness (BMF), launched by retired army major Robin Cope, who believes gyms are for slackers: the more expensive the fees, the more useless the product. What's more, last week's advice from the Government to change our daily habits by putting a bit of oomph into our DIY, vacuuming and ironing is scornfully dismissed as "pathetic". Pain, clearly, is where it's at.

This is exercise as it was meant to be: sweaty and designed to build up muscle and crunch fat in a concentrated period of time. The high after you've completed a session in the great outdoors is more satisfying than anything a £100-a-month gym can provide. At least when you get your breath back.

"All the Jacuzzis and treatment rooms in the world won't get you fit," says Cope, who spent 16 years in the army. "Gyms are facility providers. We have no facilities, but we'll get you fit. We'll take you out of your comfort zone. It's not a question of shouting and making you miserable, but we expect you to do as you're told."

This is the ultimate no-frills workout, and it's taking off. More than 1,000 BMF recruits are paying £30-£50 a month (depending on the number of classes they attend). Classes are also held in Hyde Park, Blackheath, Clapham Common, Hampstead Heath and Lloyd Park, in Croydon.

You can turn up for sessions anywhere once you're a member, even do seven days a week if you're truly masochistic. "If you want your body to benefit, you need to push yourself and feel the pain; that's what working like this does," says Cope.

He believes when it comes to burning calories, an hour with him or his team of instructors can be the equivalent of two hours in the gym.

One of my reasons for joining BMF was to strike a blow against procrastination. Getting to the gym was hard enough, never mind which machines to use. With BMF, there is no choice to be made other than turning up for an hour and picking up a bib.

Then the instructor, on this occasion former airborne officer Jeremy Wormington, will fire off a volley of instructions until those quaint evenings of warming up in a gym will seem like a distant memory.

Aside from being super-fit and supertough, Wormington also has terrific eyesight. At the end of one relay race involving running, squats and lunges, he noticed one team cheating and they were sentenced to extra pressups, at the double.

Some of us were finding all this easier than others. Emma Dowdeswell, 24, a PA from south London, has been with BMF for more than year and relishes every moment of it.

"The motivation you get is phenomenal," she says. "It's like having a personal trainer combined with being in a supportive group." Dowdeswell has applied to join the Territorial Army on the strength of her new fitness regime. I can't say I'm tempted to join her, but the BMF instructors will be seeing more of me in future.

They're the best reason I can think of for avoiding listening to Britney Spears through a pair of sweaty headphones at the local gym.