That's great you've been able to train for and compete in so many long races. I've been running for almost 50 years, but it has been about 15 years since I was able to do long runs and races.

Your rheumatologist seems awfully conservative. Usually it's the other way around. They try to get people off of prednisone too quickly. Most websites say that PMR lasts 1 - 2 years for most people, but it can last longer, Five years is pretty long for PMR. Websites say it is a self limiting disease, meaning it eventually burns itself out. If I were you I would try to get off it and see what happens. Worst case you can start taking low dose prednisone again. It would take quite a bit of time before GCA could develop to a dangerous level, and you could restart treatment before that happens.

One big issue for you might be adrenal insufficiency. After being on prednisone for so long, it might take a long time for your adrenals to become fully active again. I take weekly Actemra injections for GCA and PMR, and I also took prednisone for a little over a year, until I stopped 2 months ago. Beginning at 5 mg, I tapered one mg every 4 weeks. I didn't have any adrenal issues until a couple of weeks after I stopped the prednisone. I had some problems for about 4 weeks, with fatigue, sleep problems, and mood swings. But the problems were mild to moderate.

I was very surprised about how I felt after tapering from 1 mg to 0. I didn't expect to feel much different, but it did make a big difference in my metabolism. I felt like myself again, where I could eat as much as I wanted and not have to worry about gaining weight.

I'm very envious that you can perform so well in distance races. Enjoy the training and the races!