I am squatting in a giant wheelie bin waist-deep in freezing water with an ex-X Factor finalist. He is singing in an unnaturally high voice. I'm trying not to think about what might be happening to my feet, or that a dozen other bottoms have sat in this icy puddle today.
Two men in army fatigues are counting down the seconds until our three minutes of agony are over and they help us clamber out. If I had any breath – which I don't – I would be cursing the travel editor.
I am trying a new luxury, "boutique" boot camp. My brain – with an agility my body is lacking and encouraged by photos of four-poster beds on the No 1 Boot Camp website – leapt to images of fluffy bathrobes and scented candles.
The website also has photos of well-toned people doing frightening things in the North Sea. But that definitely won't be me. "It is boutique?" I ask, after attempts to obtain any sort of itinerary have failed and I'm beginning to get cold feet (metaphorically at this point).
"Oh don't worry," I'm assured. "We have all abilities here. We aren't one of those shouty boot camps. There's lots of stretching. And things." Then, in case I am put off, the organiser adds, "We are very effective. We really get results."
Effective? Results? Like scientifically proven face cream? Or Maoism?
A week later – eyelashes tinted and toenails painted in preparation – I am in a country house in Heacham, in north Norfolk, cataloguing my physical prowess to two men in combat gear.
"I do a lot of yoga … And I cycle to work," I add, when they look distinctly unimpressed. "It's a long way from Shepherd's Bush to King's Cross," I continue limply (these chaps have been in Afghanistan; they don't care).
"You do know what boot camp is all about," says the tall one, who looks so much like Action Man I'm sure his eyes are operated by a button on the back of his head. "There are times when you are going to be out of your comfort zone."
Going too far on the Northern Line takes me out of my comfort zone. I'm not sure which of us is facing the biggest challenge here. This is not the boot camp for softies, as billed, but a muscle-burning, blubber-melting exercise regime, starting at 6am in the morning, on a strict calorie-controlled diet.
The mission: to lose weight. Getting fit is a good thing too – but, as becomes obvious from the coos of admiration over the last group's "before and after" photos, even among the slimmest (there are some scarily sporty looking people here, already Lycra-ed up), the week's success or failure will be measured in the final weigh-out.
I should make it clear how pathologically unsuited, physically and psychologically, I am to this sort of regime. Waking me up is like bringing someone round from anaesthetic, and I'm pleased with myself if I'm at my desk by 10am. After a catalogue of humiliations, I retired from all organised physical activity at the age of 12. If there was an exam for getting out of PE, I would have got full marks for consistency and creativity. In terms of body type, I would be a jockey, but I'm afraid of horses.
And then there are the weight issues. My mum had the sort of attitude towards scales that other parents have towards sweets – unhealthy and not allowed in the house. As a result, I'm one of those irritating, scrawnyish people who take a lofty (I like to think feminist, but it is undoubtedly something far less noble) position on dieting and body image. My only food intolerance is dieters. Add an extreme – bordering on hysterical – aversion to cold weather and team sports, and I'm so far out of my zone I'm practically in space.
I consider drafting a sick note or faking some apocalypse which means I'm urgently required in the office – but I've seen what happens to dodgers in films and I don't fancy doing press-ups on someone's sweaty socks. There's no time to plan my escape, as we are told to be "kitted up" and "in line" in minutes, and anyone who is late ("adrift" as we say in the military) must pay the price. I'm still waiting for my welcoming rose tea and Indian head massage.
We begin with a run to assess our fitness, then on to circuits – a system of torture refined over centuries. We end with a bonding game of rounders. The last time I did anything around cones was my cycling proficiency. Mr X Factor sends the ball into the next field. (I offer him belated votes in gratitude for time-wasting.) I bring shame on my team, my family and my country. Not before time we are dis-missed.
But the biggest ordeal is to come: the dreaded ice bath – two words which, in any sensible person's book, go together like friendly and fire. According to the science, it prevents your muscles seizing up; it would be impossible to sustain this intensity of exercise without it, apparently. It isn't mandatory – but it is your duty. We all take the plunge – some more manfully than others.
Supper is small, tasty and as calorie-free as possible for it to qualify as food. Afterwards, we take a "bimble" down to the beach, where – quelle chance! – a distressed motorist has got her car stuck in the sand. Thank goodness there just happen to be two marines with more muscles than the local restaurants and 17 pairs of willing hands bimbling by.
End of day one at boot camp – 9pm.
We are woken each morning by blaring music to be ready for our jog on the beach at 6.30am – On The Dot. After a Spartan breakfast – filter coffee is our only toxic treat – the mornings are the toughest: variations on the circuits-team-games-circuits-running-circuits theme. One morning is spent "flipping" tractor tyres along the beach (as you do). It's true what they say about being able to do more than you thought possible.
There are times – setting at a punchbag with all the panache of John Prescott – that I worry I might be one of those people in experiments who do terrible things to rabbits just because they are told to. At others, I become paranoid that there are hidden cameras in the rhododendrons and that there is something the nice man from X Factor has omitted to tell us. There are enough tears, tantrums and group hugs to get Cowell's vote – not to mention more "journeys" than are made from King's Cross to King's Lynn each day.
The afternoons are slightly more leisurely: hikes through the – blessedly flat – Norfolk countryside or along the beach, a visit to Sandringham, a surreal trip to the local pool, reminiscent of school trips and verruca socks. We are sustained on these expeditions by an occasional oatcake with a scraping of hummus, half a banana or quarter of an apple. And seeds. A bar of dark chocolate produces the sort of effect rarely witnessed except on wildlife programmes.
The trick is to keep us moving. All the time. There is something strangely therapeutic in relinquishing all personal volition (for a limited period and – sort of – voluntarily, obviously); having one's day controlled down to the last minute, not even having to decide what to wear.
Every evening I go to bed – before dark, muscles twitching as if I've been electrocuted – promising myself that if it is raining tomorrow I can go home having fulfilled my journalistic duty. And each morning I am cursed with a glorious day. I haven't spent so much time outdoors since I was a small child, and I mean face time with grass – "Nose to the deck over there. No slacking!" Doing it in the cold and rain and mud must sort the men from the boys.
It isn't just because of the weather that I don't do a runner. Without wanting to come over all Cheryl Cole, you really don't want to let "the group" or the instructors down. One lady in her fifties keeps going all week, despite two nasty injuries, with less grizzling than the rest of us. But most of the credit has to go to our instructors, "Buffs" and "Ruby" (real names classified), both in the Royal Navy. They are a winning double-act: Buffs, all biceps and Brummie charm; Ruby, so down the line he must iron his stomach along with his T-shirt. One word of praise from either could make a girl weepy at her weights. I don't think there is a single person in the group who doesn't feel they are gunning for them.
The rest of the outfit, unfortunately, isn't quite so shipshape: the operation is definitely more "boot" than "tique". Karen and Deena (not forgetting Buddy the pet Yorkie) used to work in fashion and are lethal with a fake tan spray gun (there is a tanning tent, bizarrely, in a kitchen). Organisation is eccentric: I am not alone in my classification error (it turns out I am on "Classic" camp – mercifully it isn't the "Elite"); an unsuspecting bride-to-be and her maid of honour expected a bridal weekend; poor Danny of X Factor fame signed up to an all-male camp, but it is OK as several women thought they were on all-female ones. As it turns out, no one really minds: blitz spirit and all that.
The defence budget might have been cut, but loo roll is presumably still essential – we have to request emergency supplies. One incentive to get in the ice bath quickly is the chance of still getting a hot shower afterwards. Cleaning the bathrooms means deliveries of bottles of Cif and sponges outside each door.
But as the week goes on, these things – like mascara and Earl Grey tea – cease to matter. I'd even say they add to the charm, or at least the novelty. They know they've got a crack team – and that if people go home nearly a stone lighter (which many do) they won't care about sparkling bathrooms and plump pillows. On the pampering side, a lovely local sports massage therapist is on standby; several wouldn't make it through the week without her ministrations.
The food, by a former Oxo Tower chef, is top class. If it weren't for the portion sizes you wouldn't guess it was a diet menu. We are given boot camp recipe books to take home, which I may even use. One afternoon we are visited by the nutritionist for Chelsea football team, who has sensible things to say about the nutritional value of almonds versus Twix – whose mission to persuade his audience away from calorie counting is like trying to tell bankers not to think in bonuses. (One girl keeps a tally of numbers of Creme Eggs burnt off with a gadget on her wrist.)
Towards the end of the week people visibly shed years and pounds. As the moment of truth and the final weigh-in approach, I am as anxious as anyone that people achieve their "goals". Most have – Danny from X Factor is definitely the winner, losing 18lb. No 1 Boot Camp is undoubtedly effective.
I now have biceps ("guns") to rival Madonna's – and just as creepy. I could challenge Michelle Obama to an arm wrestle. I have lost a couple of pounds – but that is probably just my daily tea deficit. I definitely feel fitter – or I would if I could move without hobbling. (I am suffering my first ever sporting injury.)
What else did I learn? That being outside all day can be very nice; not to be fooled by fancy words for hotel; and that I'm still crap at netball.
• No 1 Boot Camp (020-8502 1144, no1bootcamp.com) runs several camps. A seven-day classic camp in Norfolk costs from £1,045pp based on two sharing (£1,095 for a single room). It also runs camps in Ibiza from £850pp a week (the next one is in February) excluding flights
