I was complaining about sore quads (because of so much mileage) to one of my bicycle mentors the other day (in preparation for the Tour of Colorado) and he said, "Rest as hard as you train, and I don't mean laying off for a day, I mean a few days".
Since I've been doing that, I do feel significantly stronger.
Yesterday I hammered a very difficult 30 mile mountain bike ride and today I am hunkered down indoors while it's drizzling outside WITH NO GUILT! As I age (64) I get concerned with how fast one's conditioning "falls off" if exercise stops.....I guess that's a whole different thing from allowing your body to regenerate!