Strength and muscle gains (or fat losses!) abound during rest periods - outside the gym. The problem with the role of recovery in exercise and understanding it is that it has not received much attention and nor has there been many articles in magazines or chapters in fitness books written on the subject to constitute any understanding on the full recuperative cycle.
I don't have to modify anything in the Insanity workouts, despite my illness and I find it fun to clench my teeth and think that every-time I push through the burn or do a punch that I punch right into the face of Multiple Sclerosis and every second I can go longer and digg deeper is a defeat for the illness itself and a victory for me.
Besides the general wish of being healthy and look good, it's the most important thing that keeps me motivated and willing to push.