Many people injure themselves because they are so obsessed with trying to reach 30 minutes of continuous exercise that they do not stop exercising when they feel pain. If you're out of shape, you should start out by exercising in each session only until your muscles feel heavy or hurt and then you should quit for the day. Eventually, you should be able to work up to a full 30 minutes.

If you can't exercise for 30 minutes continuously in one sport, try to get at least 30 minutes total exercise or vigorous activity during the day. According to the American College of Sports Medicine, you don't have to engage in vigorous exercise for sustained periods to gain substantial health benefits. If you exercise for 30 seconds, rest for 30 seconds and alternate exercising and resting, you should be able to exercise far longer without injuring yourself.

If you tire early in one sport, you can exercise in several sports, stopping in each when you feel the least discomfort. For example, go to your aerobic dance class and stop when you feel the least bit tired, even if you have to quit after two minutes. Rest, and then ride a stationary bike until your legs start to feel heavy, perhaps for three minutes. Later in the day, walk for a while until you feel tired. Try for a combined total time of 30 minutes of exercise per day, three to five days a week. You can count any physical activity that keeps you moving constantly, such as walking, climbing stairs or gardening.

Dr. Gabe Mirkin has been a radio talk show host for 25 years and practicing physician for more than 40 years; he is board certified in Sports Medicine and three other specialties.

Dr. Mirkin's daily features on fitness have been heard on CBS Radio News stations since the 1970's. He has written 16 books including The Sportsmedicine Book, the best-selling book on the subject that has been translated into many languages. His latest book is The Healthy Heart Miracle, published by HarperCollins.

Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. A Boston native, Dr. Mirkin did his residency at the Massachusetts General Hospital. He has served as a Teaching Fellow at Johns Hopkins Medical School, Assistant Professor at the University of Maryland, and Associate Clinical Professor in Pediatrics at the Georgetown University School of Medicine. He has run more than forty marathons and is now a serious tandem bicycle rider with his wife, nutritionist Diana Mirkin.