M66 here, running for 47 years, but with long breaks because of several medical operations (permanent titanium screw in my heel and other problems ...).
But I have a problem knowing my individual heartrates.
My resting heartrate is 37 says my garmin and also my cardiologist. Most formulas calculate my max hr at around 165 - 175 and my lthr at maybe around 155. So if I put this in my garmin device and do some intense hill repeats or sprints I easily achieve 190 or once even 211 ... but dr. google says the max hr cannot be exceeded.
So may I ask you to show me your max hr and lthr and age and if you also can exceed it if you "want" to.
If I let my max hr at 211 in my garmin forerunner all my normal running is of course in low aerob and mostly unproductive.