The faster you walk, the more calories you burn. For example, a 60-minute walk at 3 MPH burns 240 calories (based on a 150-lb person). Speed up to 4 MPH, and you can blast those calories in just 42 minutes. Keep it up for an hour, and you'll burn nearly 50% more calories--364 total. At that pace, walking 5 days a week, you could lose an extra 10 pounds a year.
And those are just the visible benefits. "Ramping up your speed trains the heart, lungs, and skeletal muscles to use oxygen more efficiently," explains Brian Duscha, an exercise physiologist at Duke University Medical Center. Researchers at the University of Michigan also found that women who walked at 4.2 MPH for 15 weeks increased their bone density, while those who walked 3.4 MPH saw their bone density decrease.