Joel Clark signed up for a thrice-weekly fitness class at age 46 after reeling from the results of a YMCA fitness evaluation. His father and paternal grandfather both died of heart attacks before they were 70.

Clark, now 57, fully expects to die of one, too.

But maybe not. His father and grandfather didn't run marathons, or ski with more vigor in their late 50s than they did in their early 40s. Nor did they leave younger men and women eating dust on bicycle rides.

``Doggone it, if I were to die of a heart attack tomorrow, I'd leave with the feeling that my time on Earth has been made more enjoyable because of exercise,'' said Clark. ``I feel great. It just seems to make all the systems perk along better.''

Physical fitness doesn't come with the guarantee of living to see 100. But many of the complaints we attribute to aging - muscular aches, shortness of breath, fatigue - are actually the result of disuse, say doctors. Here is the best news: You can reverse damage caused by sedentary living, and it's seldom too late to get started.

``We tend to think of the 50s and 60s as a critical time because often our levels of fitness decline as people retire and their children leave home,'' said Dr. Ed Wagner, director of the Center for Health Studies at Group Health Cooperative. ``There are societal messages that it's time to slow down or that you've earned your rest. Those are entirely the wrong messages.''

There are few injuries associated with moderate exercise, particularly if it's started slowly, says Dr. Eric Larson, medical director at the University of Washington Medical Center.

``Fitness is important at any age, but the greatest value at age 50 is that is makes people feel better. When people are strong and fit, it takes less relative effort to do the things you do every day.''

Adults traditionally are advised to seek medical approval before starting a fitness program.

Aerobic endurance and muscle mass tend to peak around age 20. Our ability to use oxygen during exercise declines about 1 percent a year after that, while muscle mass declines at half that rate. By 70, people who have been entirely sedentary or who have had illness may find they use their entire capacity just to carry groceries, leaving no reserve.

But people who exercise usually delay the decline. If they've already experienced impairment or loss of reserve due to inactivity, they can raise their capacity back to past levels. A fit 60-year-old, for instance, can have the same strength and endurance as a sedentary 30-year-old. And, adds Larson, ``the benefits will accrue regardless of when you start.''

People who exercise aerobically, that is keeping their heartbeat up to a target rate for 20 to 30 minutes three times a week, can improve the performance of their heart, lungs, muscles and perhaps even their blood vessels. Exercise can strengthen bones, lower blood pressure and improve metabolism and reaction time.

In practical terms, that means a person who exercises into his later years is less likely to have bones that fracture easily and is even less likely to fall.

And there may be more good news.

``It's still speculative,'' said Wagner, ``but there's some suggestion that older people who exercise may protect their intellectual ability.''

Gayle Brannon, an exercise physiologist at St. Peter Hospital in Olympia, finds that most of the people in her Fitness At 50 class are post-menopausal women concerned about increased risk of osteoporosis and heart attacks.

The American Heart Association says women age 50 to 54 have a heart attack rate of 1.9 per 1,000 population, compared with 10.6 for men. By age 70 to 74, the rate is 10.2 per 1,000 for women and 24.7 per 1,000 for men.

Rather than worry about heart attacks, Joel Clark channeled his concern into changing his lifestyle. He cut back on fatty foods, quit smoking and limited his alcohol intake. All of it came naturally once he started exercising regularly, he said.

Clark gave up running because of knee problems, but he has become an avid walker, a sport that gained popularity in the 1980s and is expected to be universal in the '90s. He walks almost daily with a group of former running mates.

Sports physiologists predict that as the population ages, fitness clubs will become more specialized. High-tech ``Look-At-Me'' environments will be set aside for people in their 20s and 30s. The more mature crowd will seek a comfortable setting that adds motivation through social support. More people will want low-impact exercise at clubs that offer a link to medical expertise.

Task masters spouting the no-pain, no-gain theory should be muffled, says Kim DeGallier, program director for the Providence Fitness Center. Research has proven that moderate exercise is more healthful.

For walkers that doesn't mean a leisurely stroll - you need to be ``slightly out of breath and sweating a little bit,'' says DeGallier - but it does mean there's benefit in the simplest tasks, even yard work.

There has been great emphasis in recent years on researching the benefits of fitness for the young and for the elderly, but folks in their mid-40s and above mostly have been on their own.

``I know of no program specifically targeting people in their 50s,'' said Eileen Mackle-Kern, who coordinates a senior health promotion program at Group Health Cooperative that is testing for Medicare the long-range benefits of senior fitness.

``But, clearly, everything we're telling people now at 65 should have been told to people when they were 50.''