Heat affects stamina?

I have been diagnosed with PMR. Heat and humidity really affects my stamina. On a cool day I can walk slowly for 40 minutes or so. On a hot/humid day about 3 minutes. When I see footage of marathon runners almost collapsing and their legs go all rubbery that's exactly how I feel. Anyone else experiencing this? At the moment I'm on 11 mg Prednisone and trying to taper off 1/2 mg every two weeks.