Owen Anderson in his new book "Running Science" cites several studies indicating that a running-specific strength training program can make you a healthier, faster runner. He recommends a variety of drills, circuit training, and even training with weights.
There are many sources on the Web that also outline strength training for runners. Two are: http://www.runnersworld.com/workouts/10-essential-strength-exercises-for-runners?page=single and http://www.runnersworld.com/tag/strength-training .
I personally train 3 times a week because I feel it gives me a more holistic approach to exercise.
Build up strength in your external rotators and glutes using Monster Walk drills http://triathlonresearch.org/strength-training-for-runners-how-to-perform-the-monster-walk-with-bobby-mcgee/