Ponce de Leon searched for the Fountain of Youth in hope of finding a magic potion to keep him from getting old. Health experts say he would have been better off exercising.

"You'd have to go a long way to find something good as exercise as a fountain of youth," said Dr. Roy J. Shephard of the University of Toronto. "And you don't have to run marathons to reap benefits."For the average older person who does little more than rapid walking for 30 minutes at a time, three or four times a week, it can provide 10 years of rejuvenation."

In one study, 200 men and women ages 56 to 87 participated in a program that included one hour of walking, calisthenics and stretching, three to five times a week. Results of the conditioning program yielded lower blood pressure, decreased body fat, increased lung capacity and stronger muscles.

However, all health experts warn that before beginning any exercise program you should consult with your physician. Still, most agree that moderate exercise performed on a regular basis can have positive health benefits.

The booklet "Walking for Fun and Fitness" provides information on warm-up and conditioning exercises, guidelines for distance and speed, and proper clothing. For a copy send $1 to: Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. Mention stock No. 017-001-00447-1.