When Jack Shepherd, my co-author, entered my fitness office in September 1975, he wasn't aware of how out of shape he'd gotten, but he knew he couldn't run a city block or climb a flight of stairs without becoming breathless. He wanted to begin a jogging program, he told me, because he thought he should have more exercise. He was, after all, 37 years old; friends had already died from heart attacks.

Shepherd looked fit, or almost fit. Yet in fact he was dangerously out of shape. His resting heart rate was 108, far above the normal range for men of 60 to 80 beats per minute. His blood pressure measured above normal, and I couldn't even test him on the bicycle ergometer because his heart rate would shoop up too fast.

What did we do? First, I sent him to his doctor, who gave him a stress test and an electrocardiogram. A medical conclusion similar to mine was reached: Shepherd was grossly out of shape.

At the start, like many other sedentary Americans, Shepherd couldn't run for 20 consecutive minutes. So he ran two minutes, walked two, ran two, walked two. After eight weeks, he was out of the gym and into Central Park, running a mile and then two miles non-stop. By the end of six months, he was up to five miles. His resting heart rate was down to 75 beats per minute; his blood pressure had also dropped to normal.

If you're a weekend athlete, you are probably not in shape, either. In fact, you may be in danger: Too many of us believe a round of golf or a few sets of tennis on Saturday or Sunday constitute fitness. They do not.

To be fit, you've got to run, wallk, bicycle or swim vigorously at least three times week; these are the rhythmic exercises that use the major muscles repeatedly.

Running is the heart of my program - and my life. It's cheap. It's enjoyable all year round and anywhere you travel. How fast? How far? Throw away the stopwatch, and listen to your body instead.

I define a beginner as someone who cannot run easily for 20 consecutive minutes at a steady comfortable pace, and talk while running. So, after your doctor says you may run, you're going to set aside three hours or more a week; you're going to warm up, cool down and run within your limits. And you will do this for eight to 10 weeks.

The three phases - warmup, run and cool-down - are the same for every runner. Only the time spent at each phase will vary according to your fitness level. But no runner - beginner, intermediate, or advanced - should ever skip any of the three parts.

This week we'll concentrate on the 15-minute warmup, designed to get your heart rate up. It's also important for relaxing and stretching tense muscles.

Stretching a muscle properly means stretching it slowly, not with a bobbing or jerking motion, such as bouncing downward to touch your toes.

Also, avoid working on calisthenics without letup. Exercise, then rest, then resume, then rest. Keep in motion, but don't convert any exercise, such as push-ups, into a trial against exhaustion.

Avoid deep-knee squats. The old Airborne squat jumps are dangerous. Knee injuries may result. Do not perform exercises that cause hyperextension of the back, especially ones in which both legs and both arms are raised simultaneously when lying on the stomach. "The Runner's Handbook" prescribes 16 exercises to be done in sequence. Here's a sampling. BELLY BREATHING. Lie on your back, knees flexed with feet flat and heels close to your bottom. Concentrate on letting your stomach rise as you breathe in deeply. Let go. Do this about 10 times. LEG LIMBERING. On your back, knees flexed, eyes closed. Breathe comfortably. Slowly draw one knee up toward your chest as far as you can. Slowly lower the leg back, then slowly let it slide forward and drop loosely. Return to the flexed position. Repeat with the other leg. Do this three times with each leg. Good for hamstring muscles. DOUBLE KNEE ROLL. On your back, both knees flexed, arms outstretched to your side, palms down. Roll both legs together to one side until one touches the floor. At the same time, turn your head to the opposite direction. Hold. Remain for a few seconds, then roll to the other side. Do one complete roll three times. Good for abdominal and back muscles. CAT BACK. On hands and knees, breathe in, arching your back like a cat, tucking your head in, chin to chest. Reverse, bringing your head up, expelling your breath and forming a "U" with your spine. Repeat three times. PUSH-UPS. Do as many push-ups as you can comfortably and with good form. Try five, and work up to 15 or so. HAMSTRING AND CALF STRETCH. On your back, both knees flexed, let one leg slide forward and drop. Point your toe, and slowly raise the leg up into the air as far as you can without bending the knee. Slowly lower the leg to the floor, and let it relax. Now, cock the heel - pointing the toe toward the knee - and again raise the leg. You should feel the stretch in the calf muscle. Lower the leg slowly. Do both exercises with each leg three times. SIT-UPS. On your back, knees flexed, hook your toes under the sofa or have someone hold your feet. Hands behind your head, roll up smoothly to a sitting position with your head close to your knees. Breathe out as you roll up. Roll back smoothly, inhaling. Do it slowly, carefully; don't arch your back or strain for an extra sit-up. Start with five and work gradually up to as many as you can comfortably do with good form.

Always do sit-ups smoothly, with your knees bent. That takes the pressure off your lower back and maximizes the use of your abdominal muscles. The stomach muscles should be relaxed between each sit-up, even for a brief second. Space abdominal exercises throughout your workout. BACKOVERS. On your back, knees flexed, arms at your sides, palms down by your thighs, slowly raise your hips and curl your back. Do not force. Hold the position for 20 or 30 seconds.

After 15 minutes of warm-ups and stretching, begin walking along your jogging route.