Jason Fitzgerald, from Strength Running, is convinced that variety in the types and intensities of running workouts over time is key to building endurance, strength, and speed, and I think he's onto something with this idea. He's recently released a free ebook that describes 53 different workouts for runners that can help add a little spice to the old training routine, called 52 Workouts, 52 Weeks, One Faster Runner.
The idea is simple. By using a variety of workouts, runners can emphasize different muscles groups from day to day. Workout variety may reduce injury risk immediately by allowing the muscle groups worked hardest in the previous workout, to continue their recovery, while you do another workout that emphasizes a different muscle group. It may also reduce injury risk over the long haul by helping the muscles, tendons, ligaments, and bones needed for strong running adapt to a higher workload so they can support higher intensity efforts without injury, and also recover more quickly afterward.
So, if you're suffering injuries or just bored with doing the same old workouts all the time without seeing the progress you'd really like, have a look at Jason's free ebook for 53 workout ideas (he included a bonus workout).