It seems entirely possible that endurance cycling damaged my heart.
I took up cycle-touring and joined the CTC in 1974. I joined Audax UK in 1979 and completed a Super Randonneur (SR) series in 1980. My second child was born towards the end of that year so family commitments meant that I never repeated that, although I did manage to fit in a few 200km events and a 400km in 1982. I kept up my cycling mileage, though, averaging 7000 miles per year, cycle-commuting plus club runs.
In 1987 I decided to prove that life begins at 40 by doing another SR series. Well, that didn't happen, mainly due to Achilles tendinitis, but I did manage four 200's and two 400's. That, however, was the year that I first experienced slow atrial fibrillation (AF). That first episode lasted only a few of tens of minutes, but it recurred a few months later.
Over the next few years the AF episodes gradually became longer and more frequent. 1992: started taking digoxin and a beta-blocker for heart rate control. 1994: the arrhythmia became permanent. 1995: cardioversion attempted but unsuccessful.
2010: echocardiogram showed heart rate somewhat fast and atria very large, so "warfarin is highly recommended". 2011: started taking anti-coagulants, warfarin at first, then edoxaban for the last few years. One cardiologist told me that my AF made my heart 30% less efficient, which I interpret as a 30% reduction in flow rate and hence in power, which is consistent with my average cycling speed dropping from 17mph to 13 or 14.
In recent years my annual mileage has averaged around 5000, but is now slowly declining thanks to anno domini. And I just keep taking the tablets.
So true!