Hi, Warren,
Sorry to hear about your heart attack, but I discovered, a heart attack doesn't have to be the tragedy it used to be. In fact, last year was among the best of my life, largely because of the heart attack; it allowed me to make changes and discard some mental baggages that I had needed to do for a long time. It also has helped me enjoy life more and avoid putting things off.

Regarding the high heart rate, I was told to avoid it, but I know that numerous times while hiking, biking, running, etc. my heart rate has gotten quite high, with no ill effects. But I no longer seek a high heart rate for its own sake, and in fact I've seen information in recent biking literature that achieving a super-high heart rate shouldn't be the goal but rather a lower heart rate for the same level of exertion--and that can be achieving without busting the pump. Sounds like your 20- to 50-mile rides would be perfect.

I hope this helps. As I said, a heart attack can be a beginning, not an end.
Bob Julyan