Okay, I tried that. Pulse immediately after was 76. Two minutes later, 64. Two more minutes, 52. I think my maximum rate would be in the upper 160s? So perhaps I am not doing enough to push to a higher percent of the maximum.
If you are over 50, then unless you're being chased by something I wouldn't ever try to hit my maximum. Don't break your cardiovascular system by pushing it too far. There are charts on the net that will give you a safe target heart rate depending upon age. Say 120 - 130, after several weeks of working up to that if you're in your 50's or above, and if your max is 160.
You said your resting rate is 52. Are you having any missed heart beats? Missed beats can be caused by a number of things, even too much caffeine can cause it. Many people seem to have this nowadays, but usually the heart starts hitting on all cylinders during exercise.
Another check is your BP immediately after a set, and again after 2 minutes. Charts are online that will give you an idea of where you are at by how much your pressure returns to normal.
If I were you, I'd continue what I'm doing but add a routing that get's your rate up to around 120 - 130 after slowly building up to it. Since you're in uncharted waters here, use caution and you may even wish to talk it over with your doc.
I'm sure you are in good shape, and I'm probably seeming overcautious, just do no harm . :-)