Well, most of us are not Olympic sprinters or marathoners and many of us use running as our main or only form of exercise. While this is fine, everyday runners will benefit by adding some strength training. In the December 26th posting, "Getting to the Core," I touted the benefits of building a strong core. A strong core provides good posture as well as a solid base for the rest of your body to do its job properly. When you run, the power your legs receive originates in your core and moves down to your legs.
Overall strength training can help prevent injuries, increase muscle endurance, as well as strengthen weak joints and muscles. Increasing your overall strength can also help minimize fatigue (especially in your upper body) which can help improve your endurance. Strength training is a little different for runners. A runner's strength-training program should consist of more repetitions and less weight (resistance). A runner isn't concerned with bulking up so lifting or using heavy weights isn't the key, but rather using lighter weights or less resistance, repeated 10-12 times each set, building to 3 sets is the key.
Research shows that strength training for runners should mimic muscular patterns associated with running. So, in addition to good core-strengthening exercises, exercises such as one-leg squats, high-bench step-ups, one-leg hops in place and lunges (all of which closely mimic the overall body posture and muscle mechanics of running) are good ones to add to your routine.