Unsurprisingly, leg exercises are very important while you're training to run a 5K. Building up your leg strength will help you avoid injuries, as well as improve your muscles' ability to use oxygen. With increased strength comes better posture and less stress on the joints, two things all runners should pay attention to. You can do strength exercises for your legs at the gym or at home, usually in about 15 to 20 minutes per session. Consider supplementing your strength training with a swim -- water provides excellent resistance that builds muscle without taxing your joints.