When I was a long distance runner in high school and college, our coaches encouraged us to do lots of cross-training. As I get closer to 40, its pretty clear to me how important it is to maintain core strength and flexibility through cross-training, especially if I want to stay in good running shape.
I liked this article in Runners World UK which points to the value of swimming as a great way to cross-train if you are a runner. Worth a read if you want to mix up your fitness routine!