Peroneal tendonitis

I have self diagnosed peroneal tendonitis since the end of last year. Have been managing it myself (lots of calf raises) but also stretching which I’m now reading might be harmful. I was able to run 2 half’s this year and manage the pain but now as I’m marathon training, the long runs are really aggravating it to the point where I need to rest more during the week and am missing runs. Please help with more exercises I can do and any other advice. At this point, I just want to be able to finish the marathon (my first and it’s on my birthday!) I did see a doctor who recommended an MRI and PT. Xray didn’t show any fractures