Run the Boston Marathon? Big Deal. Swim the English Channel? Ho hum.

Anyone can get motivated for Boston-Marathon fitness. Just the thought of 26 grueling miles, with hills reaching to the sky on a warm spring day, would stir anyone.

And think of how easy it is to get fit readying for the English Channel. Now that's motivation.

But show me the modern-day athlete able to stay fit while strapped to an executive's swivel chair, or a secretary, corralled in an office 8 hours a day, who can attain physical prowess pounding on a keyboard. Give me the housewife who can firm her form while balancing babies, unloading groceries, and ironing laundry, or the student who can condition his body while slumped over differential equations, and I will show you the real fitness superstars of today.

You will never read about these athletes on the sports pages nor will you ever recognize their names. Their acts of physical fitness will never receive the public kudos and accolades they deserve for staying fit in the midst of an unfit environment of elevators, push-buttons and automobiles.

These athletes aren't confined to training on a field, diamond or court. They aren't restricted to using specialized equipment. And they aren't bothered by ever having to find a partner or a team, for theirs is the pursuit of Incidental Fitness (IF).

IF is fitness through daily life style - it's making fitness conform to life style and not the reverse. Unlike the more traditional forms of training, such as running, swimming or bicycling, which might be called "Premeditated Fitness," IF is obtained almost by accident.

But like all new concepts, IF must stand the test of ridicule, then discussion and finally acceptance. Just look at all the people trotting around Washington in nylon shorts, mesh tank tops and new running shoes. Only a short time ago they were snickering at passing runners. IF, too, will have to endure the skeptics.

Every bureaucracy or business runs on paper. Notes, memos, letters and reports all have to move from one office to the other. This communication is the lifeblood of business and it's where the IF athlete can train.

(There are additional benefits to hand-carrying messages from office to office. The mail usually gets there faster and in one piece, and it's much more personal.)

But before you begin, you should understand certain training techniques. First, estimate your average pace length and whenever possible estimate and record your daily tally. You will be amazed at the effect of recording: How aware you become of fitness and distances. After awhile you automatically will know the distances traveled by estimation and experience, especially those routes you travel frequently.

The real beauty of IFF is that you never have to change into your work-out clothes because they are the same as your work clothes. No socks to wash, no gym bags, now showers. And, IF is limited only in its application by the imagination.

Take, for example, an IF athlete who travels occasionally. His wife used to pick him up at National Airport's circle: door-to-door service. She'd spend 30 minutes in line trying to get around the circle, frustrated and wasting gas. Now he meets her at a location about a mile from the airport to which he walks. The results are obvious: no frustrations, energy conservation, muscular strength (carrying a bag for 1 mile) and cardiovascular endurance.

Total fitness is a combination of strength, flexibility, and cardiovascular endurance. Incidental fitness when applied to life style can help meet all three components. And, when certain exercises are attached to particular daily habits, they are apt to be done regularly.

Another example, when you rise in the morning, instead of leaping out of bed (or dragging out, as the case may be) take a minute to stretch your back by doing "the plow" for a healthy back. When you rise, lean against the wall and do the wall stretch - see diagram - to keep the vulnerable Achilles' tendon from aches, pains, and injuries.

When you take a shower, bend slowly at the waist and let the warm water hit your lower back. This also relaxes and stretches the back. From this position go into the hamstring stretcher - see diagram - and while you're down there, wash your toes.

After the shower, sit on the floor to put on socks or nylons. Take a moment to do the abdominal curl to strengthen your stomach muscles.

Then it's off to work. Run, cycle, or walk if you can, but if you can't, try to get some incidental exercise along the way.

Don't choose the closest bus stop to your work place. Get off the bus one-half mile from your building or park your car at the far end of the lot, and walk - briskly - the rest of the way. While at work, try to keep your muscles exercised throughout the day, even while you are at the desk. The pull-up, hand press, arm curl and others will keep you fit even as you sit.

IF devotees also can redefine the coffee break and substitute a health break. Walk around your office building several times as briskly as you can. The results are cardiovascular fitness and a better outlook when you get back. If done several times a day, you can get in 2 miles without even trying.

Or walk at lunchtime and eat a piece of fruit at the desk later. That technique has a double bonus: decreasing caloric intake and increasing burnoff. If you must go to a restaurant, take a stroll rather than a taxi.

It is estimated that over 80 million Americans today are overweight, and according to nutritionist Dr. Jean Mayer, the single most contributing factor is lack of exercise.

Says Dr. Richard O. Keelor, director of program development for the President's Council on Physical Fitness and Sports, "In 1850, human muscles provided 33 percent of the energy used by workshops, factories and farms. Today, the comparable figure is less than 1 percent . . . ."

Incidental fitness may not save the world from its energy problems, nor is it likely to produce any Bruce Jenners or Frank Shorters, but it just might keep millions of Americans stronger, slimmer and healthier. CAPTION: Illustration 1, Hamstring Stretcher This exercise stretches the posterior thigh muscles without stressing the lower back. Stand and cross one leg in front of the other. The toes of the front leg, but not the entire foot, should touch the floor parallel to the rear foot. Slowly bend forward from the waist and hips, keeping you rear leg straight, heel to floor. Relax your neck and arms and bend forward as far as comfortable. Hang, breathe regularly and deeply for 20 to 40 seconds. Stretch the other leg in the same manner. Repeat twice for each leg; Illustration 2, Wall Stretch Place your rear foot flat, heel down, toes straight ahead. Lean into the wall, stretching your posterior lower leg. Hold for 30 to 60 seconds. Repeat with other leg. Repeat, this time bending each knee slightly and exaggerating the stretch even more fully. Hold 30 to 60 seconds. Repeat with other leg. Repeat both variations 2 to 5 times.