So now comes my "too late benightment": I firmly believe that the running is the reason that I am in better shape than 98 percent of 77 year old people. However, I am just as confident that my Valve Leakage was "caused" by this running. I firmly believe that WALKING on a treadmill would have prevented my present condition.
AI, as noted, it's almost a certainty that your running is the reason or the biggest reason you're in excellent shape at your age. Running is one of the healthiest things a person could do for both their body and mind. Exercise in general is great but running in particular does so much for the body. Walking, which is obviously easier for people out of shape to start with, is also incredibly healthy for the body.
With that said, anything done in excess in life can become bad for you. My unprofessional opinion is I do believe your valve leakage is from running. As somebody that also runs relatively frequently(1100 miles during COVID 2020, but usually 300-700 depending on work), I've periodically read articles, magazines and running discussions on the internet throughout the years. While I'm not on a running forum, my point is small tidbits of knowledge have been picked up over time just the same as being a member of this forum leads to one learn more about cars.
One thing that's always been noted is consistent
high intensity running will have increased wear on the heart. When I say increased wear, I'm paraphrasing but I think it's understood what's meant. While running is great for the body, there's a point that all of the health benefits will come with some negative side affects.
High intensity running is certainly relative. I'm inclined to believe your running wasn't truly high intensity given the mileage and most long-distance runners taking things slow. Still, I'd imagine there's a cumulative affect from so much running and mileage over the years.
I think one quality nearly every runner has is ambition. Running faster, running for longer distances(like a marathon or half marathon), running for a total mileage best in a single year, etc. There's a point where we're likely all being too ambitious and should 'keep it slow' or in other words keep both the distance/intensity down to something moderate. The more we run at a certain point, the more returns are diminished and likely bring other side affects.
At 28 years old, I'm likely far below the average demographic on this board. I started running when I was 17 and a complete fat ass. It is mind boggling how much better I started to feel after regular cardio workouts for a month, yet alone when I lost the weight and to this present day. The biggest quality of life turn around was my asthma. Something that used to be nearly debilitating is almost non-existent for me now.