Follow your doctors orders. Find a good PT (not one who wants to sit on the other side of the room and talk to you about exercise) and make them your ally. Talk to the PT about your guitar playing and make that a focus of your over all treatment plan. MOST important... be patient and persistant. I went through this process with bilatteral eppicondilitis. It was not quick, but I am still playing guitar years later.

This process will probably be the best way to keep developing your callouses as well. It all really comes down to how many hours you play... not how many years.