It has been called a fetish, a fad, and even a phenomenon of our times, this faney for physical fitness.

Sonny Jurgensen, now a svelte 195 pounds, takes off his shirt on the nightly news as cameras record him huffling and puffing on a treadmill, telling people how to get in shape (what chutzpah) and waxing enthusiastic about the joys of jogging, the beauty of the bicycle.

A book called "Running" has sold nearly 500,000 copies and has been No. 1 on the best-seller lists for weeks. Five of the top 15 trade paperbacks deal directly with health and fitness.

Dollars spent on sporting goods have gone from $2.2 billion in 1960 to $13.5 billion in 1977. Last year, Americans spent $257 million on running equipment.

Newsweek puts joggers on its cover, U.S. News and World Report writes about "The Fitness Mania" and New York magazine devotes eight pages to tell about "The Physical Elite: They Think They're Better Than You Are."

Clearly, sweat chic is in. A recent Washington Post national telephone survey confirmed that more Americans are doing more to get their bodies toned up and tapered down. And yet, the same poll in dicats that many also are doing less.

In the survey, conducted in February, 30 percent said they were doing more to keep themselves physically fit than they had in the past, 40 percent said they were doing the same amount and 28 percent responded they were doing less.

Fifty-two percent said they engage in excercise or physical sports in their leisure time, but 45 percent said they did not.

"No, those figures don't surprise me," said Casey Conrad, executive director of the President's Council on Physical Fitness and Sports. "Most people just can't seem to manage their lives. Look at the people who smoke. They tell you 'I know I shouldn't smoke, but I can't stop.' It's the same with exercise. Everybody knows you should do it, but they won't.

"All you can do is try to get the word out, give them the truth, hammer away at them through the media. If people don't want to accept it, well, in our way of life they can be as fat and as out of shape as they want."

"Those are rather disturbing figures," says sports psychologist Thomas Tutko of San Jose State University. "I wish I could explain it. I can just say the people I know who do less are fairly fixed in their habits. They've been smoking or overeating all their lives, and it's difficult to get out of those patterns."

"I would like to think that wasn't the case," adds Dr. Samuel Fox, a Georgetown University cardiologist and member of the President's Council.

"But I suppose I can also understand why they're doing less. There are those who have been told by their jogging friends, for instance, about the great joy of running. So they go out, they get the shoes and the sweat-suits and they try it incorrectly.

"They try and recapture their youth, they got at it almost too vigorously. They get aches and pains, sore knees or shin splints and they they say the hell with this."

Fox and Tutko, too, would prefer to talk about the positive aspects of the fitness boom.

"Cardiovascular diease is down most markedly," Fox says. "Studies are showing that middle-aged men are stopping smoking, that middle-aged people of both sexes are losing weight, that considerable numbers of people are cutting out saturated fats and of course, that more are exercising."

Adds Tutko: "The people who are doing more have a variety of reasons. They feel good just because they're doing something for themselves.

"And once they start an exercise or running program, they look upon the time they spend on the activity as their time alone, a peaceful time, a getaway time where you can let your mind go. It's very therapeutic.

"It's also had a pronounced effect on their sex lives. There's a hormonal change as well as the simple fact that people who are more active physically seem to have more energy, and they're more capable of performing sexually.

"Just speaking for myself. I can tell you why I've done more. I'm 47 now, I have had, good, close friends who have died the same age, sometimes younger. None of these people was physically fit and most of them had self-destructive habits.

"All of them were under pressure, they had no outlets for it. Exercise was not part of their lives. I just began to think I better start doing something about myself. I think you'll also find that if you do it for, say, six month, you become almost an exercise addict. You need a fix every day."

Corporate America also is discovering that better bodies also translate into better business. There now are 400 major corporations with full-time fitness directors running employe programs.

The Xerox Training Center in Leesburg features a $3.5 million facility that includes two gymnasiums, indoor and outdoor running tracks, a 25-yard swimming pool, four tennis courts, two volleyball courts and an 18-hole putting green.

Brent Arnold, director of Xerox's fitness program, says only about 10 percent of the company's 300 full-time staff members are involved in a formal lunch-hour fitness program. But almost all employees make use of the facility, open 12 hour a day, several times a week.

The federal government also is getting involved in promoting fitness among its employes. The departments of Justice and Transportation have full-time fitness directors.

"We had 312 cardiovascular retirements in Transportation alone last year," said Bernard Jankowski, who runs the DOT program.

"That translates to $2 million a year in disability cash-payment outlays. When you take into consideration how much it costs to find replacements, and train them, well, the costs can be staggering. That's what we're trying to reduce."

Other findings in The Washington Post's fitness survey indicate:

Sharp increases in physical-fitness activities among women and the elderly of both sexes.

As age increases, those who have increased their exercise levels feel to a greater degree that they are more fit than others their age.

The more money you have, the more you exercise.

There are significant differences in activity levels depending on region of the country, with people in the West leading the nation.

Those exercing more spend an average of seven hours a week on their activities.

The main activities of those exercising more are walking, jogging, swimming, calisthenics and tennis.

Those whose work involves physical activity tend to exercise more in their leisure time than people with more sedentary occupations.

"I think there's something to that," said Fox. "People are looking for careers that combine healthy living at sufficient salaries to support them modestly. The idea of being on the board of directors of six different companies is not a real goal for a lot of people.

"Whether that's in the national interest can be argued. Hard work and ingenuity are supposed to be what made this country great. If we back off now and say 'let's not break out in overtime sweat in our careers,' that may not be good for the balance of payments, but it's great for the health and well-being of a lot of people."

Tutko agrees.

"A lot of people I know will not take a second job simply because they want the time to exercise. Anyway, if you make more money, you'll have to pay more taxes. You don't get that much benefit out of it.

"What good does it do to make the money and not live long enough to really enjoy it. I'm not saying be fit and be poor, but it sure beats hell out of rich, fat and dead.