Fifteen years ago, Lt. Col. Kenneth Cooper of the U.S. Air Force Medical Corps developed an aerobic fitness program. "I couldn't sell it," he says, "to anybody."

So in December 1970, he left the military, moved to Dallas with "a pregnant wife, no job, no savings," and borrowed funds to start a medical practice devoted to wellness.

Today his Aerobics Center employs 122 people and has a budget in excess of $6 million. The Cooper Clinic has seen more than 22,000 patients, the Aerobics Activity Center lists 2,300 members--with a 14-month waiting list--and the nonprofit Institute for Aerobics Research is a leader in the emerging wellness field. Dr. Cooper's first book, Aerobics, is considered a bible of the fitness movement, and--with his three succeeding fitness books--has sold more than 12 1/2 million copies in 29 languages.

"Ten years ago I was a quack," he says. "Now I'm a consultant."

At 51, Cooper says he's "delighted" to see people practicing the fitness philosophies he's been promoting for nearly 20 years. "I recently saw a report about a Wisconsin town where the whole city of 24,000 is involved in a community-wide wellness program. I sat back, thinking, 'Praise the Lord'; that's exactly what I've been saying all these years."

His initial reaction to such reports, he admits, "is to think back to all those people who criticized you and abused you over all the years. Because I'm human. But I cannot for a moment be vindictive because I'm so thrilled to see what's going on."

It does, however, irk his wife Mildred--who traveled with him on a recent promotional tour--that "people think aerobic dance started the aerobics movement. But even aerobic dance creator Jacki Sorenson gives the credit to her association with Ken."

"None of that matters," interjects Cooper, a lean, 12- to 15-mile-a-week-runner. "What counts is that we're getting good response now."

Wellness is now "in," he claims, for two major reasons: "People see the results" and "the economy."

"By 1990," he predicts, "at least 1,000 of the 6,000 community hospitals in the country will close their doors from lack of business. So . . . they're getting into other areas like opening wellness centers."

Hospitals are closing, he says, "because the decade of the '70s saw such a tremendous decrease in deaths from heart attacks in this country. Judging from the frequency of heart attack deaths in the late '60s, there should have been 164,000 people die of heart attacks in 1980 who didn't."

People are living longer, he says, "because there's been a big change in the health habits of the American people in the past 15 years. First and foremost, there's been a change in exercise patterns. Then a decrease in cigarette smoking, better control of diet, stress management and blood pressure control.

"It's been said that when a person dies he dies not so much of the particular disease as of his entire life. People are starting to understand that. And they realize they can't afford health care--it's cheaper to stay well."

Although Cooper's basic "exercise for health" message is still the same, "over the past few years, I've shifted gears. Now I'm emphasizing six factors for health: proper weight--which includes diet and nutrition, exercise, controlling cigarette smoking, control of drugs and alcohol, stress management and the importance of a wellness exam.

"Nearly all of these relate in some ways to exercise. I've had hundreds of letters from people who tell me they couldn't break the cigarette habit until they started exercising. But exercise isn't the be all and end all. Total well-being is a condition that arises from an overall state of physical and emotional balance in one's life."

One of his most important findings in the past five years, he says, is "the abuse potential of exercise." A former marathoner, Cooper "used to feel you couldn't run it into the ground. Now I know that you can. We have identified thresholds of physical activity beyond which there is a beneficial change in the cardiovascular system as well as the level of activity at which there are detrimental effects."

Unless training for competition, he advises limiting running to "around 12 to 15 miles per week." More mileage "will greatly increase the incidence of joint and bone injuries and other ailments." Less "will fail to achieve the desired improvement in the body."

People who run more than basically three miles, five times a week "are running," he contends, "for something other than fitness." In response to the exercise addicts who take exception, Cooper replies with a chuckle, "I used to be considered a radical, now I'm the conservative."

The central focus in Cooper's new book, The Aerobics Program for Total Well-Being (M. Evans and Company, Inc., 317 pages, $16.95), is physical-mental-spiritual balance. "If a person goes too far in either direction--too little or too much exercise, food or rest," he writes, "then his or her entire physical and psychological system gets out of kilter."

The most common mistake made by beginning exercisers, he says, "is overdoing it . . . trying too much too fast. You've got to start slowly and work progressively." People past 40 or with a history of physical ailments should check with a physician and consider taking a stress test before starting any fitness program.

Among his other advice:

* Follow the "25-50-25 rule" to lose weight. Eat 25 percent of your daily calories at breakfast, 50 percent at lunch (before 1 p.m. if possible) and 25 percent at supper. To maintain weight, distribute calories "25-30-45."

* Exercise at the end of the day, just before the evening meal, to help dissipate stress. It also may aid weight loss, since exercise helps depress the appetite.

* Avoid exercise facilities with high-pressure tactics and lifetime memberships. Check credentials of instructors and the "progressive nature" of the program.

* Remember the difference between "training" and "straining." The "no gain without pain" maxim, he says, is "dangerous." Among signs of "pushing too hard": restless sleep and excessive fatigue.

* Learn to monitor your heart rate.

"Fitness doesn't make you smarter," he concedes, "but it does make you more alert and receptive. Fit people . . . cerebrate better. Studies have indicated that in conjunction with exercise, greater originality of thought is shown, the duration of concentration increases and mental response time is quicker."