This worries me as well, somewhat. I’ve read so much conflicting research/opinions on it, and i’ve seen so many healthy older athletes, but also hear stories about people who need pacemakers etc, so no idea really.
My takeaways from it are: very frequent sessions at threshold can cause desensitization of barorceptors (or something) in the heart etc. It’s the constant grinding away at threshold pace/HR that can permanently desensitize these receptors, cause chronically low HR & more stress etc.
On the flipside, infrequent sessions at threshold are fine. You stress the heart, but you give it time to recover, you’re good.
For me personally, this means training mostly at low percentages of my max HR, below aerobic “zones”. For me @ 35, that’s anywhere from 100-139. I try to keep most of my training there, especially towards the left end of it (110-120 BPM). I don’t feel any “heart stress” in these zones. If I run daily at 145-170 BPM, I can “feel” the toll it takes on my heart/cardio system. I’ve trained like that, I end up feeling like crap. So now I do lots of “ultra light” training/mileage (running), and infrequent speed/racing. Since I train mostly 2x/day now, that’s 11-12 sessions at “easy” effort, 2-3 at hard effort (threshold+, speed/racing).
I also think bodyweight has alot to do with it. I hear more stories of people having cardiac problems later on in life, from training too hard, but previously being out of shape. So people going from “weak base” to “warrior status” with lots of extra weight. The more muscle & fat you have, the harder it is for your heart to work and pump blood throughout, so I feel like this is a pretty significant factor that people rarely talk about. ie, I feel more comfortable being extremely light & putting in lots of mileage, than if I were heavier putting in less mileage.
Then there’s the “yolo” side of things. If you just love it and dgaf, then none of this even matters.
I’d personally look at it from a hunter/gatherer standpoint though, that’s how I approach things. If every day you are struggling to survive, you’re going to die sooner (I imagine). Just seems like “darwinism”. If however, you are comfortable 5 days a week, and “hunt” (push the limits) 2 days a week, just seems alot healthier. That’s some serious evo-bro stuff but it makes sense to me.