Prednisone seems to effect different people differently. Some it gives energy, and strips it from others.
Only advice I can offer is to start slow and gentle, for very short periods. You don't have to exercise for an hour solid. Maybe 15 minutes of morning yoga, 10 minutes of Pilates in the afternoon, followed by a 20 minute walk in the evening. After a couple weeks, add another 10 minute session or or lengthen your workout sessions by 5-10 minutes.
I have a DVD that provides 7 different types of 10 minute workouts for when I have a few extra minutes.
best of luck!