I was training for a marathon (to lose weight, mostly) and broke my ankle (unrelated to training). I was walking in about 5 weeks, but it took at least 9 weeks to run at all and over 2 years to run comfortably for any distance. I got a road bike for lower-impact exercise while the ankle was healing. I finally did the marathon this last September, and still run once or twice a week, but I think I'll stick with biking and only do a marathon again if I lose so many brain cells that I "think" that an Ironman is a good idea.