I had injury last year just 3-4 weeks before marathon (shin pain) and doctor ask me to rest for 3 weeks. I ask whether I can do cycling or swim. And the doctor asked : "does cycling or swiming use your legs?". So, the answer is simple - NO. I have to do upper body exercise only to keep the cardio-vascular working. (end up playing a lot of Wii Sports).
But the rest helps. I fully recovered after 3 weeks, did some warm up a few days before marathon and finish my Full Marathon.