I am in my mid-70s and was unusually active for my age before diagnosed with PMR.
I agree with "do what you can when you can".
In fact my experience has been activity, even strenuous activity can be as much an assist to relieving PMR dis-ease as it can antagonize PMR flares.
To the specific activities and the extent obviously needs a balance. But personally I am willing to push the balance over the line occasionally if the overall effect is to incrementally increase the level of activity I can comfortably sustain.
I look at it as similar to why when I drop my Prednisone dose, I do so willing accepting that some difficult flares are likely, .... or that as is the case it may be so uncomfortable that I increase the dose back again.
But I do it to get better, healthier.
I workout, hike, go surfing ... knowing occasionally I'm going "to pay for it" in the short term, but in the long term, make my body stronger and more efficient..... more able to recover.
I also believe that a certain level of activity stimulates Cordisol production. And while I fully realize (now) that as long as my body is getting 4 mg of Prednisone (I'm at 6mg now) , my body will create no or very little Cordisol on its own. I still choose to believe that continuing to keep the Cordisol producing channels "open" , will assist the body in taking back over sooner.
This is just my perspective, gathered together from several info resources (or as my wife likes to say "the world according to Stevie").

Some folks never have been active, they may have other pre-existing conditions that kept them from exercise, even before PMR.
It's not the level of athleticism you achieve. And it's not how quick you build muscle or lose weight.
It's:
Did you walk a little longer than yesterday?
Did you do some soft stretching exercices before bed?
Did you weed your garden?
.... Just are you participating in your own recovery to help your body help you?
I strongly recommend yoga for those just starting or restarting to approach being more active than they have been. Legitimate Instructors can help you modify poses so that you don't injure yourself or overdo it. Keeping muscles supple is a great help for me in shortening the duration of flares.
And I say cautiously because everyone one responds differently to different herbs, there is a specific strain of Kratom (Mitra Speciosa) sold by the Mitragaia company, called Super Maylay Green that is the only thing I've found the quickly diminishes flares, without causing a lack of normal functions (other than the pain going away there is no "high" or "drunkenness".

So yes ... I believe activity can cause flares, and I believe ultimately activity is what the body needs to stay healthy enough to recover.