If you're a regular walker looking to ramp up your workout, easing into running could be the right move for you. "Running is a more efficient way to achieve a higher level of fitness," says athletic trainer Ralph Reiff, executive director of St. Vincent Sports Performance in Indianapolis. "You can accomplish more in less exercise time." Simply put, runners burn more calories than walkers, mile for mile. Since you're working your heart and muscles harder, your aerobic fitness improves. Plus, studies have found that running lowers the risk of coronary heart disease and reduces blood pressure and bad cholesterol levels.