I ran cross-country and track all through high school and college. Then half heartedly tried to keep it up for a few years and finally quit altogether for about the last 2 years. I seemed to get injured a lot. Last year I started commuting by bike. I got my cardiovascular fitness back up a little and have not had any injuries. Now I'm getting the itch to start running again. I think I'm going to build up very slowly and try to run on days that I don't ride at first. I think patience and gradual increase is going to be the key whether you are a biker adding running, or a runner adding cycling.