(MONEY Magazine) – At 6:45 a.m., you could excuse Maitland Dimock for wondering why instead of vacationing on the sands at Maui he is huffing up the rock-studded face of a Mexican mountain, wiping sweat from his eyes and high-stepping over the occasional evidence of passing cows. You might even forgive him for being a little irritated at his wife, who lured him to volunteer for this elegant boot camp -- the Rancho La Puerta health spa in the hills south of San Diego. But Dimock's not angry. In fact, he's having the time of his life -- in part because Debbie, 40, is chugging right alongside him on the spa's morning hike. ''We were considering Hawaii,'' explains Dimock, 48, a Lafayette, Calif. industrial insurance broker. ''But my wife had been to Rancho La Puerta before, and she said, 'It's not just for women. It's very active.' '' ''I knew he would like it,'' adds Debbie, 40, ''because he's so into exercise -- running, racquetball and weights.'' The couple ditched their Hawaii plan and actually paid about $650 less -- a total of $2,200, including transportation -- for a week of sunrise-to-sunset exercise punctuated by stunningly tasty, low-cal meals at the luxurious spa near Tecate, Mexico, an easy 15-minute drive from the border. ''We love it,'' adds Dimock. ''I think a lot of people are backing away from vacations where all you do is sit around and drink.'' Dimock may be on to something. Though as yet there's no pina colada glut reported in the Caribbean, nor any fire sale on beach chairs in Waikiki, an increasing number of vacationers are choosing shape-up escapes. About 1.5 million Americans will visit spas in 1989, according to an estimate by Todd Allen of the management consulting firm Temple Barker & Sloane in Lexington, Mass., and their ranks are growing by 10% every year. But if the word ''spa'' conjures up visions of weight-loss clinics, beauty farms or Old World sanatoriums, forget it. These are sprightlier, hipper places catering to a clientele that includes celebrities from Bill Buckley to Bill Murray. The average age is younger -- about 45, compared with 55 just a decade ago -- because the fitness boom has produced a growing stream of customers whose idea of relaxation usually includes perspiration. Alcoholic beverages are scarce, but the cuisine is long on flavor (if short on fat, salt and cholesterol) -- and on the cutting-board edge. We're not talking Pritikin privation. Perhaps most significantly, the cheeks getting the facials at spas these days are increasingly whiskered -- men make up 25% of the guests, compared with fewer than one in 20 just 10 years ago. With good reason: the spas have been courting men by beefing up their traditionally female program of aerobics and beautification with more strenuous sports such as water volleyball and stretch classes geared to less-limber bodies. For regulars like Jay Novik, 44, president of a reinsurance brokerage firm in New York City, it's an ideal way to recover from an enervating routine of power breakfasts, business cocktails and missed health club workouts. Says Novik: ''Going to a spa lets me get my habits under control and return to a good exercise schedule.'' Why this spa fever? One reason, says consultant Allen, is Americans' rising preoccupation with health. Aging baby boomers, in particular, are heeding the medical evidence that working out and eating right can cut the risk of killer diseases. And even those who are more self-indulgent enjoy the catharsis of a few days of semi-sybaritic suffering. ''The good nutrition, steady exercise and break in routine have a tranquilizing effect,'' asserts Mel Zuckerman, who in 1979 opened the state-of-the-art Canyon Ranch fitness resort in Tucson after his father's death jolted him into losing 30 pounds at a spa. ''People go home feeling better than they ever remember feeling.'' Of course, a week chez Zuckerman can run up to $3,020 for a one-bedroom hacienda. But rates and services vary widely, and there are relative bargains even among better-known spas. For instance, the spas spotlighted on page 103 all run $105 to $265 a day -- a sum that's regrettably easy to spend on a big- city hotel with no meals thrown in. And you can get further price breaks by staying fewer days or going off-season. For example, a seven-day stay at the Doral Saturnia International Spa Resort in Miami costs $3,100 for double occupancy in the winter but only $2,075 for a summer package. But ambience is almost as important as price. If it will bother you to fall behind on the morning march, go to a less competitive, more laid-back place . like Birdwing in Minnesota. If, on the other hand, you are really fit and dexterous enough to enjoy working out next to preening, ballistic health club zealots from Beverly Hills and Manhattan, try the least expensive package at Canyon Ranch, typically $1,910 a week. For further details, check out guides such as The Spa Book by Judith Brode Hirsch (Perigee Books, $10.95) or The Best Spas by Theodore B. Van Itallie and Leila Hadley (Harper & Row, $21.95). Spa Finders, a New York City specialty travel agency, publishes an annual directory with updated prices and info about more than 300 locations (784 Broadway, New York, N.Y. 10003; $4.95). The firm will also make recommendations by phone (800-255-7727) and will even book your reservations, sometimes at rates that are below list.

For starters, here are some fitness-oriented spas, divided into three categories -- large, elaborate places; smaller, more intimate retreats; and spas linked with a resort that can offer alternative diversions for other family members.

WORLD-CLASS SPAS These are the theme parks of fitness, sprawling complexes given over entirely to healthy pursuits. Canyon Ranch is the Disney World of the genre, serving as many as 250 guests at once at its Tucson mother camp and 200 more at a new Berkshires estate in Lenox, Mass. Canyon Ranch offers an unmatched preventive-medicine program that includes physical exams, health-maintenance lectures and biofeedback training, plus a staff-to-guest ratio better than 2 to 1. By heading south of the border, Rancho La Puerta guests find equivalent luxury at a much prettier price (because both the peso and the Mexican staff salaries are so favorable). And the schedule of 45 activities -- including yoga at 6 a.m., body awareness at 9, T'ai Chi at 2 p.m. and water volleyball at 4 -- leaves little time for boredom. The busy curriculum also has the virtue of accommodating couples of divergent tastes. For example, Robert Paterson, 45, of Memphis, a pilot for Federal Express, is up at 6:30 every morning for the rugged mountain walk. Wife Patricia, 38, a bond salesman for brokerage Morgan Keegan, doesn't join him until an hour later at breakfast in the two-story Mexican Colonial dining hall, where they feast on a buffet with signs that give each item's calorie count (oat bran muffins, 70 calories; oatmeal with raisins and skim milk, 150). Then Pat heads for aerobics while Bob takes in a morning stretch class and weight training. After lunch, both catch massages, but the federales would not arrest them if they decided to take an afternoon off at the Agua Caliente racetrack in Tijuana. ''She does her thing and I do mine, and we get together at meals,'' says Bob.

INTIMATE RETREATS Smaller and less luxurious, these spas are serious about health but have a more communal feeling, since they serve only 15 to 40 guests at a time. At the Oaks at Ojai, in a small mountain town south of Santa Barbara, for example, the day sheet lists only about 20 activities but includes all the basics: the dawn trek, strength training in the weight room and aqua-aerobics in the pool. The kitchen is so first-rate that at 10 minutes before the 6 p.m. dinner, guests are already milling around outside trying not to look hungry. The meals tend to come in measured portions with few choices, leaving you little opportunity to overeat (typical 305-calorie menu: curried chicken with rice, vegetables, salad, fruit and yogurt for dessert). Seating is not fixed, so you may find yourself sharing a table with strangers -- which can be either fun or intrusive depending on your mood and the luck of the draw. And at night, you will probably wind down at lectures on nutrition, fitness and stress reduction, since nothing else is jumping. Decor is less George V than Motel Six. ''I figure people don't care too much about designer curtains when they are sweating,'' says Sheila Cluff, owner of the Oaks and also the Palms, 200 miles away at Palm Springs. Cluff says she tries to keep her rates low enough so that people can drop in more than once a year -- and two-thirds of her guests do. The serene natural beauty is one of the main attractions at the New Life Spa in Stratton Mountain, a ski town in southern Vermont. Owner Jimmy LeSage caps prices at $990 for six days (including two massages) by operating only from April to September in the off-season at Liftline Ski Lodge. A professional chef before he got the fitness bug, LeSage offers an imaginative menu followed by fervent, machine-gun-speed lectures on the nutritional evils of modern supermarkets and their salty, fatty, processed foods.

SPA-RESORTS These spas offer the best of both worlds: a place to shape up and eat right for those who are into it, and the option to golf, sightsee, booze and pig out for other family members who aren't. Typical is the posh Bonaventure Resort & Spa, nestled amid two golf courses and 24 tennis courts near Fort Lauderdale. One problem with resort spas is that you need the willpower of Gandhi to stay on your diet while your kids are shoveling down strawberry shortcake. A plus is that if you're there during the week, you may have the spa facilities almost to yourself, since many of the other guests are in convention sessions. In fact, New York insurance man Novik enjoyed Texas' Four Seasons Resort & Club so much that he booked his firm's next business conference there with meetings every morning and afternoons free for workouts. ''Instead of coming home exhausted, everybody came back mellow and relaxed,'' he recalls. That echoes the view of Maitland and Debbie Dimock, who are already planning their next trip to Rancho La Puerta. Says Maitland: ''You know you won't come home needing a vacation from your vacation.''