How many miles you should run in a day depends on your running goals. When running to get or stay in shape, the American Heart Association recommends getting at least 150 minutes of moderate aerobic activity per week. You can get in whatever mileage possible in this timeframe. When training for a foot race, you can follow any number of training plans. These are peer-reviewed recommendations on running and training volume for popular long-distance races: For half‐marathon runners, a high training volume of greater than 20 miles per week and a long endurance run of greater than 13 miles per week are associated with a less decline in pace.  For marathon runners, a high training volume of 40 miles per week is associated with a faster finish time than those with a low training volume of 25 miles per week.