Late on a Friday afternoon in spring. I am in a darkened hall with a sticky carpet and union flag bunting, a tiny, tired speck in a huge exercise class, in a huge holiday village on the Norfolk coast. It is my third class of the day. Usually I consider three lacklustre stints in the gym in a week a major achievement. I can't work out if something has gone very badly wrong or very, very right.
It feels unquestionably wrong to begin with, on the train to Great Yarmouth. My highly organised companion unpacks lunch for two from her rucksack and I discover how terrifyingly little you know about someone until you see what they think constitutes a viable meal. Wholemeal pitta bread, homemade houmous, salad and raw carrots sit in appropriately sized Tupperware tubs before us. I don't howl with despair but the anguish is clear in my eyes.
"We're going on a fitness weekend," says Helen firmly. "We should start as we mean to go on."
We are indeed. For the past five years an essentially sadistic outfit called Leisure Plan has held Fitness Fiestas - three days of aerobic, strength, endurance and flexibility classes billed as "the UK's biggest fitness party" - in holiday camps at off-season Weston-super-Mare, Camber Sands and Hemsby. They are aimed, says organiser Andrew Lucas, "not necessarily at the total non-exerciser, but from the infrequent exerciser through to the aerobic animal." This weekend - and this sentence becomes increasingly unreal to me as I type - 1,400 of us will gather in Hemsby's branch of Pontin's to get through as many of the 122 classes on offer as possible. This is a veritable smorgasbord of fitness opportunities, with charmingly evocative names such as Hot Blackcurrant Jam III, Lotsa Squats, Core Blimey, Chillates and Ceri's Balls. And all for the bargain price of £45-£89 per person.
I forgot to set my alarm for the first morning - a relatively unsubtle Freudian slip - so we arrive an hour late. It means we miss our first class, Fame ("an easy to follow aerobic dance class that will set free the Leroys in all of you. Sweat bands and leg warmers are optional!" according to the blurb, with what I suspect is the first of many misguided exclamation marks).
Happily, it gives us time to tour the site and observe our fellow inmates, who seem, reassuringly, to run the gamut of "infrequent exercisers" ("I go to the gym once a week, but this way I get to leave my husband at home for the weekend") to steel-bodied fitness addicts. Best of all, we discover a room full of bouncy balls the size of space hoppers. No matter how old you get, this phenomenon is always cheering.
My mood improves even further as we watch the end of a "Pure Energy" class being run by a man clearly unafraid of the wholehearted embrace of a stereotype. "Oo," he says, shivering delightedly as he jumps back up on to the stage. "Every time I get up here, I feel just like Judy Garland in A Star is Born." A brief medley follows, to muted reaction from the aerobics multitude. "Straight people," he chides. "Ignoramuses."
At 3.45 it is time for SS on SS Step with Kardy Laguda in Lunar's Bar. (I never find out what the SSs stand for.) Lunar's Bar is a dark and reeking hole and I find it disconcerting to exercise on 80s pub carpet as the overspill from the small central dancing area requires, but Laguda (resplendent in rainbow clown trousers and matching bandanna) is a genius. My usual step class experience goes something like this: step up, step down. Try to add arm movements. Fall over and leave. Takes about 10 minutes. By the end of SS on SS Step, however, I am twirling, sidestepping, kicking, leg curling, and frequently in the right order. I am unbelievably pleased with myself and hurtle with Helen on to Girl Power Circuit. This is in Nelson's ballroom, which stinks like Lunar's Bar and is at least as unattractively carpeted. The ceiling is strung with union flag bunting, but an uplifting sense of patriotism cannot counteract a surfeit of lactic acid.
I manage half the circuits - tricep curls, shuttle runs, squats and star jumps - but balk at burpees, those accursed inventions that require you to jump, squat and do something peculiar with your arms all at the same time. Helen lasts through some lunges but then her legs fall off and she, too, concedes defeat. But after a brief rest in our chalet and a chat with our neighbour, a veteran of six Fitness Fiestas, a curious thing happens - I become fired with exercise zeal and set off for Yoga Fusion. Helen follows, drawn, she tells me, by sheer amazement at the sight of me striding enthusiastically towards anything but a Radio Times and a takeaway.
Unfortunately Yoga Fusion is full so we sneak in at the back of Body Combat (the usual punching, sidekicking and embarrassing noisemaking) and then sneak out again to start queuing for tomorrow's tickets. This involves proximity to the Pontin's amusement arcade, which is when I discover that, in addition to a penchant for rabbit food, Helen has also been hiding a fairly serious gambling problem. Every time I turn around she is feeding more coins into the waterfall machines and bending down to watch their fate. She looks like a demented and increasingly penurious woodpecker.
While she is draining us of funds, I book tickets (too late for Fightklub Aerobox and Pontin's Rocky Horror Show, alas!) and develop a ravenous hunger. This develops into a raging insistence on eating fish and chips on the beach, to which Helen agrees surprisingly readily. It turns out that this is because to get to and from the chippy one must run the gamut of Hemsby's tourist attractions - fruit machines interspersed with fruit machines.
Then it's back to Pontin's for a shower and fitful sleep before an 8 o'clock class in the morning. I expect such an early start to kill me, but it turns out that the Fiesta atmosphere has worked on me more deeply than I knew. Not only do I manage an hour of As Easy as ABC aerobics but when I can't do Euphorobics (accidentally booking a very difficult Level 3 class and failing, despite the instructor's exhortations, to even fake it), I hare off immediately to find an alternative I can sneak into. I don't know where I end up, but I complete an additional 40 minutes of bending and stretching before joining Helen for bacon butties, congratulatory conversation and the forcible removal of her wallet into my safekeeping when she notices that the dining hall has one-armed bandits.
Then more classes, including Fluid Intervals, which sounds vaguely disgusting but actually refers to nothing more distasteful than aqua aerobics, and by the time our power-walk up and down the dunes is cancelled because of bad weather we have the bit so firmly between our teeth that we stride out together anyway. Only when we are atop a sandy tor does a sudden weariness descend.
We have enjoyed ourselves to an unexpected extent at what we had assumed would be a weekend marked by far from stoical endurance. By being immersed in a world suffused with exercise and enthusiasm, I have worked harder, achieved more and felt better than after any solo gym session. But I cannot make it through Sunday. It is time to go home. I am exhausted. Helen concurs. She is bankrupt. But, incredibly, we are sorry to go and vow to come back. Who knew that out-of-season Pontin's would be the place where miracles happen?