DALLAS -- Dressed in designer jogging shorts and/or thick Aerobic Center robes, the majority of the patients at the famous Cooper Clinic in Dallas, pantheon of preventive medicine, look a bit edgy.

All eyes are cast downward, reading a magazine or filling out a health profile, and the waiting rooms are so quiet you could hear a Rolls-Royce shift gears. You may know that lonely feeling from your own doctor's waiting room when you've been sick.

But here, very few people are sick.

They aren't giggling because of the primal thought a doctor's visit forces upon us, particularly if the visit is for a really thorough physical -- we are mortal and frail and one of these days we are either going to die quickly, or become ill and die slowly, or simply wear out. Whatever our efforts to the contrary, the circle of life eventually gets smaller and smaller and the physical measures that circle.

So what's to be happy about being here?

"Oh, I'm motivated to keep my same time," says Ginny Michaux, 46, of Alexandria. She's talking about the dreaded treadmill test, the report card for your heart and general fitness level.

You can't cheat that thing, a reality that has kept Ginny exercising.

"I have a good bit of performance anxiety myself," says her husband, Dick, also 46.

"No one asks you how your hearing or field of vision were," Michaux says of two other tests in the physical. "They ask you about the treadmill." Why do people ask about that? Because it measures your durability as a human being and because you control it, make it better or worse.

The Michauxs make it better in fine style, too. He used to work out in the missile compartment of nuclear subs at sea ("the only place you could stretch out"); she now leads their high-altitude hikes in Vail, Colo., and Diablerets, Switzerland, near Gstaad.

But even this fit couple are put in their place by more active types at times. Ginny says they once met in Switzerland "a group of nursing home candidates, I thought. Over 80 {years old}. But these people had just climbed a very steep 10,000 or 12,000 feet, and they didn't even look sweaty. Dick and I were panting and puffing."

But the Michauxs' health seems to be improving with age, too. In 1987, Dick went 27 minutes, Ginny 21 on the treadmill. In 1989, each added a minute to their time, and both moved up in their "superior" standings for their ages and sex. Not bad at all since that treadmill becomes faster and steeper as the seconds tick by.

Their doctor and mine at the clinic does even better than that, however: Arno Jensen never thought of fitness until he turned a very out-of-breath 40. Jensen had chest pains simply rushing to everyday tasks.

"Someone once said the best way to live a long life is to get something wrong with you and take care of it," Jensen says. "Those chest pains are the reasons I finally started thinking about me."

For the last 21 years Jensen has lifted weights and ridden a stationary bike daily. This week he jumped on the treadmill on a dare and went 28 minutes, the 99th percentile for any age group.

Jensen is an optimist when it comes to the slugs of the world. Whether you've given up on your health from sheer sloth or from the fear of bad genes, he'll make you feel good.

"Poor health," he says, "doesn't come from one factor; it's a synergistic problem. Even though you may be genetically inclined to one problem area like high blood pressure or high cholesterol, you have control over many factors. Manage those factors, and you'll manage the problem. Our bodies," Jensen concludes, "have a tremendous ability to recover from misuse."

That's just what I need to hear as I step on the treadmill myself after a summer of no formal exercise, careless eating habits and island rum drinking. Back in 1985 my body was 31 percent fat. I could have substituted for a seal at the circus.

By '87 the blubber had receded to 21 percent. This morning it had crept up to 24 percent. Jensen's body fat, incidentally, is an impressive, really disgusting 8 percent.

My cholesterols weren't as good after my summer of partying, either. In 1985, my total cholesterol was an unhealthy 238. In 1987 it was an okay 201. Now it's back up to a slightly worrisome 220, and my VLDL cholesterols, the real bad guys, are double what they should be.

And then there's my triglycerides level, a good indicator of health troubles to come. In 1985 it broke the meter at 355. In 1987, after exercise, no booze, and half-sensible eating, it was a healthy 139. Now it's back up to 262, and the number brought a slight frown to Jensen's face.

But on the treadmill my body, off the edge of health as it was, still performed up to my 1987 level of 21 minutes, barely in the "satisfactory" range. I worked a lot harder to get there, of course. In 1987 my pulse went to 178 and my systolic blood pressure (the top number) to 160. Now my heart beat went to 185 and my systolic blood pressure to 230.

My cardiogram was nonetheless "excellent" under all that stress, and my overall health, according to Jensen, was "satisfactory but far from optimal." A lot better than I thought he would say.

Why, I wanted to know, wasn't it worse? Jensen thinks several factors helped: Until June I had lifted weights steadily since 1987, and put on 12 pounds of pure muscle. "It's becoming more and more apparent that there are some metabolic effects of strength training," Jensen says.

And then I'm a generally more active person than I was several years ago. Though I don't exercise formally as much, I move a lot faster and look for excuses to be active.

And finally I've changed my eating habits over the past four years. Though I've been bad for three months, I had been relatively good before that, modifying my eating habits for the better rather than radically changing them.

Which brings me back to my first thought of this day: Physicals make us uncomfortable because they make us face reality. If we're lucky, they then make us do something about reality. And even a little something can bring big changes in your health.

The very reason you should think about a checkup yourself.

Checking You Out

Getting a physical: Most doctors or walk-in clinics can give you a physical. Many local hospitals operate "wellness clinics," too. At the minimum, your doctor should:

Take a complete health and medical history;

Examine you from head to toe;

Take blood and urine samples.

Getting a stress test; Find a doctor or testing center that specializes in cardiovascular testing. (Health clubs don't.) Your doctor can recommend one. At the very least, make sure your testing center always has a doctor on the premises and uses an EKG machine with a minimum of 7 leads.