It seems intuitive for endurance athletes to train almost exclusively using cardiovascular exercise.  The truth is endurance athletes need to have a balanced training program which includes strength training for improved movement efficiency, enhanced performance, and reduced injury.
Strength training can help you run faster, longer, and more efficiently. A study published last year in the Journal of Strength and Conditioning Research showed that runners who added three days of resistance training exercises to their weekly program not only increased their leg strength, but improved their running economy or efficiency.  This means they were able to run at their desired race pace for longer durations with less effort or even increase their race pace.  The added strength also increased sprint speed, giving them the kick often needed at the end of a race.
Getting in the gym and lifting weights not only increases strength, but will also increase your joint stability which can reduce the risk for repetitive stress injuries. Lower body exercises are particularly important when it comes to reducing injuries around the knees and hips, two of the most problematic areas for runners.  Incorporating exercises such as squats, deadlifts, and lunges into a workout may help prevent these lower-body injuries as well as speed up the recovery process after strenuous runs.
An additional benefit of strength training in an endurance athlete’s training program is the maintenance or the addition of lean muscle mass.  The addition of lean body mass raises your metabolism and keeps your body burning more calories after a workout and at rest.  This helps maintain optimal weight for both competitive endurance athletes and recreational runners.