When Zachariah Blackstone, an active florist in Washington at age 106, was asked the secret of his long and healthy life he had one answer: "I owe it all to exercise, especially jogging. When I wake up I think I'm dead. Jogging brings my blood pressure up and I feel like a man. I'm ready for the whole day."

Blackstone is not alone in his observations. In fact, the benefits of exercise, especially for older persons, is a subject that has been addressed by such groups as the President's Council on Physical Fitness and Sports and the Administration Aging.

The council and the aging administration point out the need for exercise in "The Fitness Challenge," a booklet outlining an exercise program for older persons.

"Advanced age need not mean inactivity or infirmity," the booklet states. "For those who are physically and mentally active, it can be a time when long experience of life enriches each passing day. An eminent physician, commenting on the phenomenon of aging, has said: 'Most of us don't wear out, we rust out.' Disuse is the mortal enemy of the human body."

That the road to physical fitness is open to elders has been demonstrated in the Adult's Health and Development Program at the University of Maryland. The program is described by its director, Dr. Dan Leviton, professor of health education, as one of "physical involvement with social interaction." About 50 elderly persons participate in the Saturday morning program every semester, each working under the guidance of a trained student volunteer.

The program includes individual and group activities such as card games (small-muscle coordination), tether ball, trampoline, swimming and dancing. A participant's physical condition and progress are monitored by the staff, and a physician's approval generally is required for participation.

As participants progress in their activities, which are individually selected and paced, there is often amazement at their accomplishments. It is not uncommon to hear a participant say, "I didn't know I could that!" What is clear is that greater body management, agility and stamina are achieved than most participants thought possible.

Research findings also contain encouraging messages for older persons about physical exercise. The Research Digest, published by the President's Council on Physical Fitness and Sports, reports that most older citizens believe (erroneously) that their need for exercise diminishes and eventually disappears as they grow older, and that they vastly exaggerate the risks involved in vigorous exercise after middle age.

Other research has concluded that a person's general learning potential, at any intelligence level, increases or decreases in relation to an individual's degree of physical fitness; that there are positive relationships between physical and motor traits and social characteristics, and that regular physical activity, while it does not always prevent heart attacks, decreases the likelihood of such problems.

In summarizing studies on exercise and aging, the Digest concludes that research clearly indicates "older people are trainable in respect to physical fitness components, even though having been sedentary for many years. Exercise, of course, cannot stop a person from getting older, but, when properly applied, can retard the aging process and add vigor to living throughout life."

Physical fitness now is perceived in a far broader sense than the muscle-building programs common in the past. Dr. Robert N. Butler, director of the National Institute on Aging shared this broader view at a recent national conference on Exercise in the Elderly. His definition, besides sound muscle and bone structure, included such things as balance, flexibility, posture, endurance and appropriate blood pressure.

Herbert A. De Vries, an exercise physiologist at the Andrus Gerontology Center in Los Angeles contends that exercise programs for older people should maximize the rhythmic activity of large muscle masses and that the natural activities of walking, jogging, running and swimming are best suited to this purpose.

One group that follows this prescription is identified by the name Masters swimmer. This organization is nationwide, enlisting men and women from age 25 to over 80 who train regularly and enter national - even international - swimming competition.

In a report entitled "Exercise and Aging," the President's Council on Physical Fitness and Sports lists some general principles of exercise for older people:

Exercise should be adapted to the individual's exercise tolerance (not so little that no effect is noticeable nor so much that distress results.

The exercise plan should provide for progression (gradual increasing of exercise intensity and duration.

Individuals must want to improve their physical fitness and thus make exercise a part of their daily lives.

The same report strongly advises that sedentary older persons, particularly, check with their physicians before starting a vigorous exercise program. Organized exercise programs such as those of recreation departments or the YMCA are recommended because they are likely to make provisions for ensuring appropriate safeguards.