Supplemental (Core/Strength) Training
During the summer, you should do supplemental training at least 4 days a week. This includes yoga, strength training, and/or plyometrics. This type of training help to make you a more efficient runners, and it can help to dramatically reduce your risk of injury. I have included links to informational articles and videos. Do not be afraid to mix it up; make sure to try many different styles of exercise - you may find that certain exercises work better than others for your body type. Develop a training program the fits with your schedule and the equipment the you have available.
Articles:
Strength-Training Circuit For Distance Runners
Get Stronger In Under 20 Minutes A Day
Adding Relative Strength Is A Must For Distance Runners
10 Essential Strength Exercises for Runners
Strength Training Program for Runners
Videos: