When it comes to physical exertion, says athlete and sports commentator Diana Nyad, "many women are tigers in lambs' clothing.

"Even when they decide to really get into an exercise routine or a sport, they hold themselves back -- for a variety of reasons -- and deny themselves the intensity the sport demands and their bodies crave."

The 32-year-old marathon swimming champion calls this reticence the "Feminine Mystique of Fitness" -- an attitude rooted in early training that "ladies don't sweat" and "muscles are for men only."

"Women have had all these myths laid on them that it's not natural for them to be strong. Which is a lot of b.s. A tiger suffers physically and emotionally when caged. And human beings -- women and men -- are no different."

Nyad has been fighting female fitness stereotypes most of her life. As a 10-year-old in Fort Lauderdale, Fla., she began getting up before 5 a.m. 365 days a year, Christmas included, for swimming practice.

"While teachers and peers admired my dedication," she admits, "they definitely considered my pursuit of the physical boyish."

She ignored taunts and jeers and went on, as an adult, to accomplish numerous firsts, including swimming Lake Ontario north-to-south in 1974 and, in 1975, swimming around Manhattan Island in record time.

That same year, jellyfish stings and severe weather conditions forced her to abandon, after 79 miles, an attempt to swim 103 miles from Cuba to Florida. But four years later she became the first person to beat powerful currents, sharks and jellyfish in an 89-mile, 27 1/2-hour swim from the Bahamas to Florida.

Nyad, who has retired from her swimming career, did not swim across large bodies of water just because they were there. While training for her Cuba-to-Florida swim she told a reporter she "felt like the Diana who represents freedom to the women waiting on the shore." Freedom, she added, from the "bonds of athletic ineptitude.

"I'm talking about fitness that makes you feel competent and good about yourself. The kind of body training that carries over into other parts of your life . . . that builds confidence and character . . . so you can walk down the street and feel 'Hey, world, here I am.' "

But this attitude is foreign to many women, she says, "partly because they're so geared to worrying about what men will think. Hey, men are vain, too. A man in a health club will stop to look in the mirror and check himself out. But he's going, 'Take a look at me,' while a woman doing the same thing says to herself, 'Oh my God, look at those thighs.' "

Which is why Nyad says she wrote (with co-author Candace Lyle Hogan) Basic Training for Women (Harmony Books, 191 pages, $13.95). "Women need encouragement," she says, "to go against cultural myths."

While her 30-minute-a-day program is "just as applicable for men, the tone is geared for women . . . (who) grew up with a trip laid on them that men don't have to deal with.

"This book is my locker-room speech to women who want to make their body stronger and enrich the quality of their life. It's written like a coach talks, to psyche you up and keep you going.

"Basic training is a lifetime involvement. Muscles will get sore. You'll sweat. But you'll have more energy, flexibility, sleep better, look better, feel better."

Male fitness experts, she claims, tend to "tell women how to get skinny, which I find offensive . . . Men are allowed a wide variety of styles that can be attractive--they can be intelligent, like Woody Allen, or macho like Clint Eastwood. But women are supposed to fit into a certain skinny mold."

Nyad, 5-feet-six and 130 pounds, spends at least an hour a day skipping rope. If she's not working on a book, her ABC sports commentary, speaking engagements or a new sports magazine format show on ABC, she puts in up to five hours a day on some physical activity. (She is ranked 12th in the U.S. in squash and aspires to play on a professional racquetball tour.)

The basic training program, outlined in her book, includes 5 minutes of stretching, 2 minutes of aerobic warm-up, 15 minutes non-stop aerobic activity (swimming, running, cycling, skipping rope), 5 more minutes of stretching and a 3-minute leg-strengthening exercise. Twice a week, a 30-minute weight-training routine is added. Total time per week adds up to about 4 hours, or about 30 minutes a day with one full day of rest.

Although "I'm not a nutrition expert," Nyad includes a short chapter on diet because "I couldn't totally ignore it." Her own diet includes a breakfast of 4 eggs, bran or wheat cereal, fruit and a quart of juice, fruit or yogurt during the day and a large late meal. ("Last night I ordered two fish dinners at 11 p.m.")

When she was in training she'd eat 24 ounces of raw steak -- cut in wedges and rolled in soy sauce -- and six raw eggs, sucked right from the shell. She has never even tasted cola or coffee. "I've had ginger ale when I was sick."

Diet and exercise are particularly important to women today, she stresses, "because now that women are wanting the same lives as men they're getting the same diseases as men, like heart attacks. This fear of ill health is in the back of people's minds. Basic training can help them do something about it."