Calf pain when exercising

I started feeling the symptoms of PMR in February after a bout of sinusitis and a stressful work situation which included being cold a lot of the time. Wasn't diagnosed until April when I started on prednisone which helped enormously. Have been on a slowly decreasing dose since then, and am now on 4 mg. When I was on higher doses I was reducing 1 mg every two weeks, but now under 5 mg the plan was to reduce 1 mg every month. But unfortunately, my symptoms have come back enough on 4 mg that I'm not going to reduce any more at the moment. Not what I would call a flareup, but just increasing stiffness and pain. It's manageable but uncomfortable. Previously I was in very good health for my age and very active-I'm 67 years old.

But my question is about an unusual symptom I have which my rheumatologist says isn't from PMR. When I walk uphill (which is my main form of exercise) even on a slight grade, I will have a burning sensation in the backs of my calves like lactic acid even though I am not exercising enough to generate that. this will spread to an aching in my ankles and even soles of the feet. I don't have pain in my calves at other times. I mainly experience the pain and stiffness in my hip area, middle back and upper biceps. has anyone had anything similar to this?

and what is confusing about this is when I exercise by riding a bike, even quite vigorously, I don't have the same pain. It's just not as easy to get exercise by bike riding where I live, whereas uphill walking is readily available and offers beautiful views. I live in West Marin, California.

I have really appreciated finding this bulletin board and all of the helpful and kind advice available from fellow sufferers. Thank you all.