C'mon, kids. The issue is that if you overdo it too early, you'll be screwed out of doing what you love later in life. That's not a good outcome in my book. OTOH, an old poll of TdF racers found that something like 80% of them would take something undetectable which would guarantee them a GC win, but also would guarantee they would die 10 years later. So there's that.
In fact, old GC champs are mostly still in good shape. Old Mr. Universe champs are still extremely fit.
I do know a few older cyclists who've gotten Afib and had to slow it way down or quit entirely, all from doing too much with too little recovery. The secret is complete recovery every once in a while, and good recovery during the season. These friends of mine did not overtrain. They just overworked their hearts, too many intervals, too many weeks in a row.
I've been training hard for over 20 years, since I was 50. I recently had a couple of heart exams which found that my heart is in great shape except for some plaque in a coronary artery, but who knows when that was laid down. So I take a statin now, but I don't worry about it. I've eaten plant-based for 40 years and my cholesterol is actually pretty good. I do major hard rides in the mountains every year and train appropriately. I've become one of the fastest riders my age in this area, mostly by not aging out rather than from talent. So it's some genetics (that plaque), and some overdoing it, which is preventable.
BTW, I regard a hard 60-80 mile ride to be defined as averaging over 90% LTHR or over 80% MHR. I ride that hard quite frequently, but I recover well between them.
So never quit, train hard, but train smart. If you don't, you won't be able to.
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Results matter