Some interesting info about sustained (long term) high endurance exercise. Negative coronary results were obtained from young rodents on a short-term (two weeks) demanding exercise regimen. They were able to bounce back and reverse ill effects on their heart. Future research is needed to measure reversibility effect on older long-term endurance athletes. Until then all things in moderation
https://www.sciencenews.org/blog/scicurious/slow-heartbeat-athletes-not-so-funny
http://www.athletesheart.org/2013/02/elite-triathletes-and-heart-problems/