Once you've gotten warmed up, you're ready to move from walking to running. It's important to begin this phase with a properly relaxed and stretched body. The transition to aerobics is achieved by moving from warm-up exercises to a brisk walk to a slow jog and then settling into your running pace. Aerobics means "promoting the supply and use of oxygen."

The main principle of aerobics is that your body must exercise at a rate that demands large amounts of oxygen for a sustained period of time, preferably at least 20 to 45 minutes three times a week. To arrive at aerobic fitness, we should bicycle 30 miles a week, or walk 15 miles, or swim 1 1/2 miles per week, or and this is really the simplest - run a total of six miles a week.

In my Run-Easy Method, aerobic exercises are based on maintaining a "steady state" heart rate for at least 20 minutes. Runners should follow two simple guidelines in governing their pace: First, their heart rate should level off at a "steady state" well within their training range; second, the pace should let them pass the "talk test." That is, carry on a conversation while running; you pace should not leave you breathless. If it does, you are working harder than your muscles can adapt to the new stress of running.

As you run, you'll have to keep in touch with your heart. Most veteran runners can easily tell their heart rate, but a beginner will need to learn to check his heart rate at rest, during the warmup and several times during the run. (After eight weeks or so, this becomes second nature and checks need not be as frequent.) Keeping in touch with your heart means taking your pulse, the number of heart beats per minute.

1. Resting heart rate: The average resting heart rate for men is 60 to 80 beats per minute; for women, 70 to 90. A very fit person's rate may be at 60 or below; marathon runners "normally" have rates in the 40- to 50-beat range. An individual's resting heart rate will drop after a few weeks of consistent aerobic exercise. Check in the morning. Count the number for 10 seconds and multiply by 6. If you find your heart beat higher than usual one morning, you may be overtraining and should cut back.

2. Maximum heart rate: This is the point at which the heart "peaks" out. It can't satisfy the body's demand for oxygen and can't beat any faster, at or near the point of exhaustion. Physiolocists have devised a method to estimate your maximum heart rate: Subtract your age from 220. This is not a goal - merely a figure to obtain your training heart rate.

3. Training (exercise) heart rate: This is the aerobic heart rate, the rate that provides sufficient training effect for your cardiovascular system. This "target zone" of safe, beneficial training falls between 70 percent of your maximum heart rate and the cut-off figure of 85 percent of maximum heart rate. It really isn't complicated.

First, subtract your age from 220. Your target, aerobic heart rate is 70 percent of this number, and your cut-off (slow-down) rate is 85 percent.

During your run, if you are a beginner, occasionally stop and take your pulse. Take it within 10 seconds after you stop to get an accurate count. Keep moving a little if you can. If you are over the target zone, walk. If you are under it (the 70-percent figure) increase your pace a little.

To make the transition to aerobic running, start walking briskly and stretch your legs and arms ahead of you as far as you can; reach out with them. Now jog slowly. Be sure that you run lightly on your heels, not your toes. Run heel-toe, much the same as walking. Running on your toes may cause shin splints or calf pain. You will soon learn to "rock" on you heel - hitting the ground heel first, then toes. This may feel strange at first, But it will soon become very comfortable. Jogging slowly, if you are not winded, you should be able to talk to your running partner or hum a tune if you are alone. If you can't, slow down or walk - but walk briskly enough to keep near your training heart rate range.

Beginners will want to walk, run, walk, run, walk. They should follow a simple pattern: Run until you are ready to walk, walk until you are ready to run again.

Most important, keep a steady pace throughout. Keep moving. Don't finish with a sprint. Avoid rapid running with high knee lift. Both greatly increase oxygen debt and heart rate, which is dangerous for beginners and intermediates. Finish your run with consistency and ease. You should complete the run exhilarated, not exhausted.

After the warmup and the run itself coms the cool-down phase, aimed at relaxation and stretching. THis is the last, but equally important, step of the three-hphase Run-Easy Method. It is the easiest to omit. But when runners skip the cool-down, they frequently get injured because they haven't stretched and relaxed their muscles after a run. Inexperienced joggers tend to dash out of their homes, run and dash back, heading straight for the refrigerator and a cold beer. The only thing I approve of is the cold beer.

Generally, the cool-down is the warmup in reverse. After running slowly, jog and then walk for perhaps five minutes. Follow this by slow rhythmic stretching exercises and relaxation, repeating some of the exercises from the warmup. The purpose of the cool-down is to return the body to its preexercise level, insuring the return of the blood flow to the heart from the extremities, and preventing muscle tightness. It is also essential to get your heart rate returned to normal. All horses are walked after vigorous exercise for these same reasons.It makes horse sense for you to do the same.

How long should your workouts be? Here is a helpful guide, with the first figure being the number of minutes of each phase for a beginner, the second for an intermediate runner and the third for an advance one:

Warmup - 10 / 20 /30

Run - 20 (minimum) / 30 / 30 to 60

Cool-down - 10 / 10 / 10

Totals - 45 / 60 / 90