While many older people know that they should exercise, and in fact feel better when they do, putting their good intentions into practice is another matter.

People over 50 now constitute over half of the U.S. population, and are one of the fastest growing segments. In addition, the 65-and-older group increases by an estimated 1,000 people per day. Between 1970 and 1980, the number of people over 65 increased by 27.9 percent.

But most do not exercise.

While 38 percent of people over 65 said they had walked in the past two weeks, only 27 percent of those over 65 and 31 percent of those ages 45-65 said they exercised or played sports regularly, according to recent data from the National Center for Health Statistics. For all ages, those who exercise regularly is 41 percent.

"It's not all that simple for an older person to exercise," says Jack Hughes, legislative chair of the Alexandria Council on Aging. "They may not have a swimming pool or bike path near them . . . and I think the inertia is a problem. They need to be stimulated to do things.

"The social mores that tell people to slow down, take it easy, don't go out of the house, while well-intentioned, can add up to not taking any risk in terms of activity . . . and a very passive existence. It happens in the people I see. We've been programmed by work. When you're no longer on a job, you have to be a self-starter."

When the flesh is willing to work out but the spirit is weak, try these motivation suggestions: Establish specific days and times for exercise. Stick to the routine. Work out with friends. "We use activity as a vehicle for developing friendship," says Dr. Dan Leviton, director of the Health Development Project at the University of Maryland. "The social factor is all-important." "Get a fitness buddy," recommends author/athlete Millie Brown in "Low-Stress Fitness." "If you're 50 and noncompetitive, don't choose a 20-year-old racer. Select someone who is compatible with your goals and schedules . . . Take out a short-term membership in an exercise studio and pass the word around that you're looking for companions to join you on walks or bike rides. There are a lot of people we come in contact with who want or need to exercise regularly but need someone to help motivate them." Be aware that you don't have to -- and probably shouldn't -- do everything shown in a class or on a videocassette. Instructors should "make it clear that they do not have to do everything," says Don Zuckerman, leader of Dancers of the Third Age, whose members range in age from 62 to 88. (You have to be at least 60 to join.) "We respect the limitations. We ask people, what is your creative approach? It's a socializing experience. I've learned that any kind of movement that has flow and creativity is good. Technique is not everything."