A bunch of my friends training for upcoming races are suddenly suffering from bizarre injuries. Some ask me for my unqualified diagnosis. Sometimes it's easy, but most of the time I have no clue. I do have some good advice though, because I've come down with a couple of injuries as well. And that never happens to me.
When I was training for my first Ironman, I bought a book to guide me through it. The author insisted the most important thing about training, is rest. I thought he was crazy. Made no sense. But he was right. He claimed we need at least one day of rest each week. That's when our body gets a chance to repair the muscle tissue we're tearing apart by all this rigorous exercise. He went on to say there's nothing wrong with listening to your body and taking more than one rest day per week if you have too.
I'm convinced my body isn't repairing itself as quickly as it used to. I'm hobbling around with a bad back and bad knees but I notice when I apply ice, I'm fine. This appears to be a case of a body nearing 50 years of age that's not used to 17 mile training runs, or even an hour bike ride every day. We need rest more than we need training. Sounds crazy but it works every time. The hard part is convincing our training-addicted brains that it's OK to kick back more often.