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Several of the people I exercise with have ended up having serious cardiac issues that seem to be related to endurance exercise.

I always try to be careful to take off enough rest days myself, but maybe that doesn't even matter and it comes from cumulative adaptation?

A bit of a meme, but why do endurance bikers look so old?

I'd go with very low fat percentage. I had a friend who ran marathons regularly, and he looked older because he was superlean and wiry.

Regular people can look older too just by losing weight rapidly: https://www.drugs.com/medical-answers/ozempic-face-3572731/

Long rides in the sunshine are rough on your skin especially if you aren't constantly applying sunscreen.