I'm 73 and resting HR is 58.

Meds can make a difference, as can wellness state.

As stated previously, recovery rate is important. Sometimes when I have pushed to near my max effort on a hill, I will slow down and stop, take my pulse - I never wear a HRM any more - and take it a minute later - and another minute later. There are some tables/charts which will help you determine what a good recovery rate should be.

This is a very gross approximation, but it will give one at least a clue - i.e., if your recover rate after one minute is a very small drop in pulse rate, then you might want to check with your doc.