Exercise burns calories. In the case of walking and running, the calories you burn depend much more on the distance you cover and your body weight than on your pace. This table shows calories burned per mile of walking or jogging on the level for people of varying weights:

A hundred or so calories a mile might not seem like much, but they can add up to better weight control. For example, a 2009 study of 4,995 men and women found that the average American gains about 2.2 pounds a year during middle age. But during the 15-year study, people who walked gained significantly less weight than those who didn't; the more walking, the less weight gain. And the benefit was greatest in the heaviest individuals. For example, walking for just 35 minutes a day saved a 160-pound person about 18 pounds of flab over 15 years of aging                                                 

Walking has it all. Simple and natural, it doesn't require any instruction or skill. It can be a very modest form of exercise or it can demand enough skill and intensity to be an Olympic sport. You can walk alone for solitude or with friends for companionship. You can walk indoors on a treadmill or outside in the city or country, at home or away. You can get all the benefits of moderate exercise with a very low risk of injury. And to boot, walking is inexpensive. All things considered, Charles Dickens got it right: "Walk to be healthy, walk to be happy."