To answer the OPs specific questions, at 58 my RHR is currently 48 (it has been as low as 43, and bounces to around 51 if I spend a month or so doing only light training). I don't know my max HR but it is certainly above 180 because I saw 181 a few months ago when chasing a kid up a hill. My lactate threshold HR is currently 158.

I train for racing, so may be one of those at risk of the structural changes referred to in the article cited by Terex. I am, however, unconcerned. In the first place, the Swiss professional cyclists were just that - professional cyclists - and they will have trained at volumes much higher than mine for many years. In the old days the pros did monster miles in training, many more than their equivalents today. In the second place, even with the increased incidence of VF, the long-term endurance exerciser tends to have lower mortality than the general population. In the third place, there seems to be very little data on the long-term impact of HIIT, which increasingly is supplanting very extended strenuous sessions for training purposes and which appears to have very beneficial short-term effects on various metabolic markers.

I think the "making your car last by not treating it like a rally driver" analogy is stupid. Your body is a biological system, not a mechanical one. It responds to stress not simply by wearing out, but by adapting and getting stronger. Your heart can become more efficient - increased stroke volume etc. - while remaining structurally normal. What kills it is depriving it of oxygen by sitting around and letting your vessels fur up.

The bottom line is that not engaging in exercise is much more dangerous than exercising, and that long-term vigorous exercisers tend to live longer and healthier lives even though some of them develop arrythmias. How much exercise is enough, in terms of longevity and retention of full function, is moot. I'm not aware that anyone has an answer. Personally I have tried both very extensive moderate exercise - five or six hours a day on the bike at touring pace, six days per week, for months at a time - and the shorter, more intensive training I do for racing, which is more like 12 hours per week but with sgnificant time spent above my LTHR. Both seem to result in my feeling conspicuously well. What that actually means in terms of health is imponderable, but the last time I had a reasonably comprehensive set of blood tests the results were described as "pristine".

I'd suggest an attitude of insouciant unconcern. If you are exercising regularly and going hard enough to get out of breath some of the time you are almost certainly doing yourself good. Very, very few of us will be going hard enough for long enough to put ourselves at risk of hypertrophy, and even those who are, are having fun doing it and are unlikely to die as a result.