Interestingly enough, a random couple things happened when I was between 43-44 that lead to me start distance running and run my first full marathon about 4 months before I turned 44.
I was decently fit before, but didn’t run any distance above a maybe 5 miles. I do not remember anything negative happening around that time or shortly after. I was actually increasing my cardio endurance dramatically over the next 5 years or so.