"I'm a heck of a lot slower than I was, but I'm still enjoying the sport," says Burfoot who has recently discovered that taking walk breaks during his runs allows him to train longer with less discomfort. More people now combine running and walking, he says, and use running as part of a varied fitness program. Instead of running every day, they run a few days a week and do strength training or walking or other sports on alternate days.

"Increasingly people are doing Tae Bo and everything else that comes along," he notes, "But in the end, millions stick with the simplest, least expensive, most efficient way to get maximum fitness in minimum time, which is by running."

While walking is also easy, low cost and effective, it takes more time than running does to achieve similar results. For example, a 130-pound adult burns about 130 calories in a half-hour walk and 300 calories in a half-hour run. The fitness benefits of walking vary depending on whether you're strolling casually or marching briskly. But running -- even at a slow pace -- is guaranteed to be a vigorous activity, with all the corresponding cardiovascular and metabolic benefits -- and some risks.

Sedentary males over 40 and females over 50 (and younger people with risk factors for heart disease) should consult a physician before starting any vigorous activity, such as running. People with joint problems may be advised to avoid running altogether or to alternate it with swimming or other low-impact sports.