The man or woman who takes time to jog or swim or bicycle several times a week to feel better and perhaps live longer may have reaped yet another benefit -- a financial one. In the past year, at least three life insurance companies have begun offering substantial discounts to the physically fit.

The three firms say the research they have studied indicates people who exercise regularly live longer.

"Although a definite link between physical fitness and longevity has not yet been scientifically established," says Donald M. Fordyce, chairman and chief executive officer of The Manhattan Life Insurance Company, which began offering the discount nationwide in May, "the most recent data present us with a probable trend that is strong enough to support lower insurance rates for those involved in appropriate fitness programs."

This latest evidence that the insurance industry recognizes that some people try harder to live healthier lives follows a long-established policy of 30 insurance companies of offering discounts to non-smokers because the evidence indicates they live longer too.

"It's a trend we want to encourage," says Linda Ozier of the National Jogging Association in Washington.

In the case of Manhattan, both the physical fitness and non-smokers discounts of about 15 percent each can be obtained. For example, Manhattan offers a $100,000 annual renewable premium of $309 for a 40-year-old man. If he meets the exercise qualifications, the premium is $268. If he hasn't smoked in a year, the cost is $226, or an overall savings of $83.

Two other firms selling a variation of the physical-fitness policy are Unity Mutual of Syracuse, N.Y., which is licensed in the District of Columbia, Maryland, Virginia and 17 other states, mostly east of the Mississippi, and Occidental of Raleigh, N.C., whose discounts are available nationwide except in New York and Hawaii under its newly formed affiliate Financial Fitness Inc. Other companies are studying similar discounts.

"I think that it really is time for the insurance industry as a whole to offer consumers a discount for physical fitness," says Manhattan's Fordyce, who is 43 and, fittingly, a lifetime non-smoker and a runner who covers "three miles four times a day in less than 24 minutes."

Despite the "great increase in physical activity" in the country, Fordyce says, "More than half of American men and women lead a very sedentary life." The insurance industry should "encourage good health by rewarding those people who take care of themselves."

To that end, his company reimburses employes who successfully complete a "SmokEnders" course. And it is sponsoring a physical-fitness outing Aug. 25 with several distance walks and runs. Fordyce is entered in the three-mile run. But, he adds, "I've got to tell you the bad news with the good."

Of the 600 employees participating, he's the only three-mile runner in the over-40 category."They phoned me up to say I'd already won the trophy. But that told me something. We'd better be doing something further for our own staff."

To obtain a Manhattan physical-fitness discount, the applicant must participate in some aerobic exercise for not less than 20 minutes a day, four times a week. Fordyce says the company reserves the right to request a confirmation letter from a YMCA or YWCA, if that's where you exercise, or from your doctor. "If you do jog, he's going to know it," says Fordyce.

Fordyce says it's too early to tell how the physical-fitness program is being received. But he said that as of June, 63 percent of the company's policies were being sold with the non-smokers discount.

Manhattan also has a parallel health awareness discount of about 15 percent to applicants who can submit proof of at least two complete annual physical examinations, the last one within the past 12 months. "People who see a physician regularly are better risks than those who do not," he says.

Unity Mutual offers a discount of up to 8.9 percent for the physically fit on its permanent (or whole life) policies of $25,000 or more. For a 40-year-old male, the standard premium on a $25,000 whole life policy is $633. With the discount, the premium is $580.

"People who are involved in jogging, swimming and bicycling live longer than those who aren't engaged in them," says Unity Mutual's Edmund Sullivan, himself a runner. The applicant must give evidence that such exercise is a part of his or her life style, see a doctor regularly for examinations and must not smoke. Verification, in most cases, will be through a background investigation. The discount program "is going very well," says Sullivan. "We're very pleased." Unity has recently sponsored a "Run for Life" race in the Syracuse area and maintains an exercise room and provides fitness lectures for employes.

Occidental was the first company to offer physical-fitness discounts, according to Financial Fitness president Tom Smith, who says the company introduced them last summer after sponsoring track and field meets for the over-40s for the past five years.

"We saw guys running in their 60s and 70s. We decided that people who keep fit not only have a better quality of life but they live longer," says Smith, 39, also a longtime runner.

Occidental's discounts amount up to 20 percent on whole-life policies and 25 percent on term policies, says Smith, and are available to applicants who have jogged or completed other aerobic exercise for 20 minutes a day, three times a week for the past year. For a 40-year-old male, the standard Occidental premium for a $100,000, 20-year decreasing term policy is $426; with the discount, it is $354.

To qualify, as with Unity Mutual, you also can't smoke. "We don't feel you can smoke and be physically fit," says Smith. A physical is required unless you have participated in a 26-mile marathon in the past year.

A low pulse and proper weight are evidence an applicant is exercising as claimed, he says.

According to Occidental's statistics, a person who takes part in at least one aerobic exercise regularly, doesn't smoke and follows good health habits can expect to live up to 11.5 years longer than non-exercising smokers.