Re: Next step
You should be fine sustaining things for a while, as the heart will strengthen and be more efficient. There's a calculator here:
https://www.active.com/fitness/calculators/heartrate
This site has a guideline to maximum heart rate:
https://www.mayoclinic.org/healthy-l...y/art-20046887 In short it says "The basic way to calculate your maximum heart rate is to subtract your age from 220. For example, if you're 45 years old, subtract 45 from 220 to get a maximum heart rate of 175. This is the maximum number of times your heart should beat per minute during exercise." If this guideline is accurate you are doing fine.
As for your age, well, I'm getting fit again at 62 after spot of heart trouble in October last year, and the Beta Blocker I'm on lowers my heart rate and makes the heart beat less powerfully as well. It makes it tricky because I don't know what my heart rate would be without taking it, but I got them to half the dose a few months ago as my resting heart rate was down to 45 or 50 beats. That seemed terribly low to me, but with the reduced dose it's now it's around 65 which seems a lot better. When I am working out in the gym it generally peaks at around 75 to 80, after I build up a nice sweat and feel the lungs pumping. Like you, I don't get out of breath. I don't think I'm much use as a comparison to you though because of the Beta Blocker.