I can't fathom your doc's advice. I'll bet a good bit that a sports doc that cycles would advise differently. You might not want to get to your max heart rate....but 4 times a week excessive? That's crazy. Even for a heart patient. 70miles in 8 hours? Even with some stopping that's not a very fast pace. I concur with getting a second opinion and a heart monitor. Keep in mind the medical background we all have here in this forum...