I know several people who hike and have had heart attacks.

I suggest that you learn a lot more about your ailment before you start making decisions. I would consult with a qualified physician ASAP (this coming Tuesday?), undergo whatever diagnostic tests that are suggested and then make your decision based on knowledge rather than hope. I would certainly hope that a stress-test would be among the diagnostics recommended.

Maintaining a high overall level of fitness year around is the best way to minimize the risk of heart attack when on the trail. This, of course, presumes that you are cleared by your physician for hiking.

To me, a sufficiently high level of fitness can be maintained by 40 minutes of aerobic exercise, at least four times per week at 65%+ of your maximum heart rate.

Good luck and let us know what you learn!