You can still reap the benefits of exercise, even if you already have heart disease, diabetes, high blood pressure, or cancer. "Most chronic conditions can be helped by some form of physical activity," says Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at Tufts University.

Heart health provides a dramatic example. After someone has had a heart attack, regular aerobic exercise over the next few years lowers the risk of another episode by between 15 and 25 percent. That's nearly the same protection provided by commonly prescribed drugs like beta blockers and aspirin, says Steven Keteyian, program director of preventive cardiology at the Henry Ford Hospital in Detroit. Heart patients are at higher risk of heart attack as they exercise, so moderate intensity exercise like jogging or swimming may be better for them than intense spurts of activity like sprinting or shoveling snow.

Studies also show that exercise can increase the amount of oxygen that congestive heart failure patients can absorb during a treadmill test, an indicator of whether the heart can keep up with the body's demands. Some patients improve enough with exercise that they get off the transplant list, says Kerry Stewart, director of clinical and research exercise physiology at Johns Hopkins University School of Medicine. Along with losing weight, exercise can help those with diabetes, too. Its effect on blood sugar levels can even help prevent the disease in some people on the cusp of developing it.

Less fatigue. The evidence on whether exercise can prevent a recurrence of cancer is less solid, but there's little question that it can help patients manage the side effects of treatment and feel better generally, says Pamela Massey, director of rehabilitation services at M. D. Anderson Cancer Center in Houston. Massey has even had her patients walk slowly on the treadmill while being infused with intravenous drugs after a bone marrow transplant. Recent research shows, too, that weightlifting, once thought to exacerbate lymphedema--the painful arm swelling that affects some breast cancer patients--is safe, allowing them to continue strength training to prevent osteoporosis. Meanwhile, a recent study published in the journal Cancer found that six months of twice-weekly weight training boosted both physical and emotional well-being of breast cancer patients. And a pilot study presented at the American Society of Clinical Oncology meeting earlier this month showed that both breast and prostate cancer patients who followed a home program of walking and resistance training had less fatigue during radiation treatments and improved their strength--after just four weeks.

Exercise also provides a huge mental lift. Karen Thibeau, 51, who runs about 4 miles four days a week with a group of neighbors, continued to do so as much as possible during her recent rounds of chemotherapy for breast cancer. On days when she didn't feel up to running, one member of her group would walk with her instead. "If you can keep up with your routine, you feel like you are the same person as before," she says. Holly Gannoe, now 35, was diagnosed with stage 3 melanoma last year, just as she was gearing up for triathlon season. After surgery to remove the melanoma, she had a year of interferon treatments and kept walking, riding her bike, and running, depending on how she felt. In fact, she ran the Marine Corps Marathon last year amid her treatments, which were just "like an endurance race," says Gannoe. "You just keep going until you get to the finish line."