I walked on the treadmill, sorry to imply it was anything else. I also am 55 and I think your worries are sound but slightly overblown. The repair is universally agreed to take about 12 weeks for you body to solidly attach it to the bone. The 0 to 12 week period's activity is wildly debated. I had a 75% tear and like you I was functional, so I delayed it for 3 years because of your fears. That complicated my progress. I am convinced I will be 100% withing 6 months from OP. You are not the type to not work hard at rehab. You will do fine...
The 1st 6 weeks, sleep loss, being confined in a sling are a mental test, but worth it. Please keep me up on your progress. My right was my operative arm, I did swap my mouse hand to left, and now I'm can't switch back, I've become ambidextrous.