<p>If you do find yourself injured (like myself) or unable to run for whatever reason, try swimming. I try to go 3 times a week (along with a pull-up, push-up, sit-up, and row routine in between) and it has done a lot in maintaining my lung/heart stamina. Its also helped relieve some of the frustration/anxiety of not being able to train in running.
I would concur with the "get the right shoes" advice. Part of my problem is that I just discovered that I have slightly flattened arches, and when that was combined with a screwed up hip it turned into a stress fracture last fall.</p>