Fill your casual downtime with walking, and you can quickly build a versatile, pain-free fitness program that burns fat, builds muscle, speeds metabolism and instills a priceless resistance to a host of serious diseases.

Walking is nature's nearly perfect exercise. Suspend disbelief, and consider that in a 35-minute walk, you can burn 90 percent of the calories you do in a in a 13-minute run — with minimal chance of injury. If you weigh over 150 pounds, you can boost those numbers by about 65 percent for every 50 pounds.

What's more, consider that one-third of fatal heart attacks in midlife women are the result of physical inactivity, and a study of more than 70,000 female nurses found that brisk walking for at least three hours a week reduced a woman's risk of heart disease by 30 to 40 percent.