Running a half-marathon may be your goal, but in order to build strength and endurance without injury, you've got to start off easy. Run a few times a week, starting with low mileage, and walking if you need to. Gradually build up to running more, following the 10 percent rule which states: never increase your weekly mileage by more than 10 percent of the previous week. For example, if you're running 10 miles per week, the following week, run 11 miles. Slow and steady wins the race, or rather, keeps your in the race!