A bespoke fitness retreat isn't for the faint-hearted, says Lucinda Labes. As she discovered in Morocco's High Atlas, if the yoga and kick-boxing don't get you, the lack of a good cuppa might

Here today, gone tomorrow. At least, that's what I'm hoping, as I feel my thighs wibble-wobble to the top of a goat track in the Atlas mountains. Cheered on by a team of health and fitness professionals, I am prepared to believe that my one-week retreat based at a health spa in Marrakesh is all it will take to bring about a total body transformation. But as any gym bunny will tell you, if you want fast results, then easy doesn't do it - you have to put in the work.

In:spa is a company specialising in fitness breaks in exotic locations. Having joined it in North Africa, I quickly discover its all-consuming programme doesn't take any prisoners. The retreat's villa, Jnane Tamsna, in Marrakech's posh Palmeraie district, may be an Arabian Nights fantasy - jasmine-scented courtyards, domed ceilings - but the daily itinerary still begins at dawn, when shots of invigorating mint tea and beetroot juice are served on the hotel's rooftop. The rising sun invites you to stretch your eyes to the horizon and the Atlas ranges bristling in the distance but there's more important stretching to be done on the rooftop. It's time to limber up for the first hike.

Vigorous morning walks are the cornerstone of in:spa's programmes; walking at high altitude not only oxygenates the blood, but tones the legs and burns fat. And the views, along a new mountain route each time, are glorious. Walking through the mud-walled villages and valleys reaching to the Sahara desert are sufficient to distract you from your aching calves. Within the first few hours on day one, the differing fitness levels of our party soon become apparent and each group is given its own guide. I soon realise that if I want to walk and talk (I do - the other guests are great), I will have to settle for the slowcoaches.

By noon, the African sun begins to sear, so it's time to head back to the ranch for a lunch so healthy the food fairly glows on the plate: organic salad from the garden; grilled vegetables; home-baked barley bread; barbecued chicken and pear kebabs. Sugar, wheat, caffeine and dairy products are strictly off the menu and as for alcohol....

The afternoons are as action- packed as the mornings; if the hikes don't get you, the one-on-one yoga, kick-boxing and personal training sessions probably will. Fitness instructor Jamie Baird has me wading the swimming pool with weights on my arms and legs, an exercise to test the cardiovascular system as well as tone the limbs. The next day, he makes me run repeated laps of the hotel; along outdoor corridors, haring over the roof, 10 press-ups in the courtyard, until I discover muscles I never knew I had.

Such a focused pursuit of health may sound like taking your body in for an MOT but in:spa's retreats are designed to give your body a whole new engine. To this end, programmes are designed to revitalise from the inside out, an approach which employs not just exercise but massage and a personalised nutritional regimen. So, while it's more than likely that you will boost your fitness levels and lose weight, you can also expect to detoxify your liver and calm your mind.

But the road to inner peace can be a bumpy one. Some guests, for example, react dramatically to the diet; during the week I was there, a fair number suffered headaches brought on by caffeine withdrawal or by the sudden reduction of their wheat intake. And while the body is adjusting to its new fuel supply, it also has to cope with a heavier exercise load. The regular walks may be easy to begin with, strolls along flat plains and dry riverbeds, but by the end of the week, you're tackling two-hour uphill scrambles before bounding back down through boulder-strewn chasms. On some nights, it's all I can do to stay awake through supper.

Thank goodness then, for the team, a group of health experts of shining good humour. The yoga teachers, Liz Lark, Jean Hall and Simon Low, may not be everyone's idea of A-list celebrities, but each enjoys a devoted following in their own right and each teaches "dynamic flow" yoga, which can accommodate both beginners and experts. The personal trainers, on the other hand, are a no-nonsense bunch who know how to squeeze the last press-up from tired arms. And then there are the nutritionists, a lean tribe who watch your diet like hawks. The massage is another godsend. Having had your muscles astonished into a state of stringy tension, it's blissful to have them soothed back to softness by deep tissue and aromatherapy massage. At the far end of the retreat's garden is a traditional hammam, a type of sauna where a Moroccan siren awaits to scrub and sluice your skin on the steaming tiles.

Halfway through the week, we are treated to an evening off for some retail therapy in the medieval Medina, followed by supper in one of the city's lesser-known riads. Even here, there is no cheating on the diet. As we sit salivating beneath the stars, waiters emerge with tureens of white fish - steamed. Their appearance prompts a group groan.

Our stomachs full, if not our appetites sated, we head back to the villa and take some consolation in the seven-acre garden. The villa's owner, Gary Loum-Martin, is an ethnobotany professor whose matchless enthusiasm has created an oasis of rare vegetables and plants: moon-pale marrows clamber up palm trees; hedges of lavender and flowering onions adorn the swimming pool area. His masterpiece is the central courtyard, a froth of fragrant white flowers. At night, the mysterious clerodendron gives out an unforgettably sweet scent, like honey, chocolate and poppies, crushed up together. Loum-Martin is happy to walk guests around his own patch of Eden explaining the whys and wherefores of his creations. Just be sure not to ask about his irrigation system; there is, after all, a limit to the amount one needs to know about boreholes.

In such idyllic surroundings, it's easy to stomach a bit of hard work and, even if you find the dietary regime a little austere, if nothing else you at least see a quick return for your efforts. By the end of the week, I'm pleasantly surprised at my overall sense of well-being; I almost feel recharged enough to run all the way back to London.

On our last night we celebrate with supper on the rooftop as below us, the garden sparkles with 100 candle-lit lanterns. The table is heaped with Loum-Martin's white flowers and a trio of Berber musicians sing to the skies. By way of a final test, we're even offered a farewell glass of wine but by now I am feeling so thoroughly virtuous that I decide to leave starting my "retox" programme until at least a week after I get home.

Detox rocks: where and when to chill out

A week at in:spa's Jnane Tamsna country guesthouse in Marrakech starts at £1,950 per person. Prices exclude international flights but include airport transfers, food, accommodation, fitness and yoga classes and nutritional consultations. Dates for retreats in 2006 are: 4-11 January; 2-9 February; and 1-8 March. In:spa also hosts retreats in Ibiza and mainland Spain, as well as in India, France and Thailand.

In:spa donates £35 of the guest fee to the Global Diversity Foundation charity.

For more information:

0845 458 0723,