Hey Pooks,
I'm glad to hear that you are excited to get out on the bike but you should keep a couple of things in mind. Shingles is a type of viral infection that is running unchecked. So although the prednisone will keep the pain down your body still has to do the work to beat the virus. So, like all other viruses that stress your immune system, some light exercise is good but too much will slow your recovery. Feel free to exercise but keep your heart rate low. If you don't have a HR monitor, try to exercise at a level that allows you to continue to breath through your nose, this will keep your intensity level in check.
Take care and recover fast!