Tom, I'm 62 and had my bypass a couple of years ago. Like a lot of guys my heart's OK (very slight damage) but it was the arteries that were the problem. I own a HRM and a Blood Pressure Monitor. I used both quite a bit after the event just to learn how my body operates - you get to see the effect various activities have on your body. It can also teach you how to lower your BP/HR with your mind and your breathing - kinda interesting. After awhile you can set your own parameters based on how you feel.

Now I use it occasionally just to see where I'm at. I've set a personal max of about 170 - I don't feel distress at that level, but I get a tiny warning that "that's enough" and back off. Cruise at 125-130, hills at 140-150.

Incidentally, are you on a Beta Blocker? Since the operation my resting HR runs a little high, so I'm on a small-dose twice a day (although I often skip the evening one if the HR's normal). They're a bit limiting, depending on how long before exercise you took one, but I find I can 'blast through' it if I really want to push myself.