The "deathly fatigue" issue is something that many have trouble dealing with. We all were more active previously, to one degree or another and struggle adapting to the "new" person we've become. The best example I read once was:

We take prednisone to eliminate or reduce the inflammation & pain caused by PMR. The prednisone in part is very similar to the cortisol produced by our adrenal glands. Our adrenal glands sense that there's enough cortisol (prednisone) in our system, so they quit producing it whenever needed. That's okay until we try to do too much or work too hard and our supply of cortisol runs out for the day. The adrenals already sensed you had enough in your system so they aren't going to fire up and create more on a moments notice. Balancing how we exert ourselves can help avoid the fatigue but it'll likely not let us do things at the level we used to do things.

Once we successfully (and very, very gradually) reduce our prednisone down to zero, we still may face the challenge of "waking up" our adrenals again. They've been asleep for a long time and need to realize that they need to go to work again! The fatigue and exhaustion once again are mainly coming from the lack of cortisol/adrenaline/prednisone in your system. At least that's how it was explained to me and I feel it makes perfect sense.

I have had PMR for 6 years and at age 67 still work a full time job. However, I don't work at nearly the level that I used to. It's a constant challenge to pace myself and work at the level allowed. Push too hard and pay the price of total exhaustion. Reduce prednisone dose too quickly and pay the price both with increased inflammation and fatigue. Most of us will get down to zero prednisone someday and have normal functioning adrenals too. How we adjust to the "new us" will determine how long and comfortable the journey will be.