The number of miles you'll need to run to lose weight depends on your size and your goals. You must burn off 3,500 calories to lose 1 pound. Running a 12-minute mile burns approximately 100 calories -- slightly more for larger people and slightly fewer for smaller people.
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Rate of Loss
If you use the 100-calorie-per-mile estimate, it'll take 35 miles of running per week -- or 5 miles per day -- to lose a pound per week. Run 17.5 miles per week and lose a half of a pound per week. This level of weight loss will only happen if your calorie intake before running was just enough to support your weight and you don't add extra snacks once you start regular exercise. You can even lose a fraction of a pound each week if you run just a mile or two per day, but this is unlikely to satisfy your dream of a fit, lean body.
More Exact Calorie Calculations
Larger bodies burn more calories than smaller bodies. For example, a 155-pound person running a 10-minute mile burns about 124 calories per mile while a 185-pound person burns about 148. A 125-pound person burns 100 calories per mile running at this pace. If you are an efficient runner and have been doing it for years, you may burn fewer calories than a newbie if you both are going the same pace because your body is accustomed to the activity and knows how to conserve energy.
A study conducted by Duke University researchers published in a 2005 issue of the "Journal of Applied Physiology" found that it took people eight months of running 20 miles per week to lose 14 pounds. The participants did not make any changes to their diet, but the running still helped them reduce visceral and subcutaneous belly fat.