On average a person who weighs around 180 pounds burns in the region of 100 calories per mile walked. A person who weighs closer to 120 pounds burns nearer 65 calories per mile.
How much a person weighs and the distance walked are what determine the burned calories per hour. The walking speed is less important than these two factors, although walking a mile in 13 minutes or less burns more calories. About.com suggests that beginning walkers focus on increasing distance before speed.
In order to lose weight, a person must burn more calories and/or consume fewer calories. Taking walks helps increase the daily activity level to burn more calories.