It sounds like your PMR isn't completely under control yet if you're having trouble walking and lifting your arms. How long have you been taking prednisone? It can take a few days for it to take full effect. I would think once your pain is under control you could start lifting light weights and running slowly. Just be careful not to do too much at first. The prednisone and the PMR can both affect your muscles and tendons and make it easier to cause strains and tears. I learned that from experience. I have PMR and GCA, and have been running for almost 50 years. I went through a bad stretch tapering from 20 mg prednisone down to 10 where my muscles were straining very easily while I was jogging and lifting weights. But everyone is different. You might not have any problems.