I think it all depends on what your max HR is. At your age I suspect it's 190 or higher but everyone is different (your real max may be higher or lower than typical 26yo).

I'm not a doctor but I'd say if heart failure while exercising is a serious concern for you (family history of heart issues?) then you should get a real fitness/stress test from your doctor. Find out what your real max HR is and discuss the appropriate HR to target for sustained efforts over multiple hours.

Personally I target ~75-80% of my theoretical max for long sustained efforts (4+ hours) at a good pace mainly so I don't feel completely spent at the end of the ride. For shorter efforts I feel fine pushing ~90% of max but my legs typically say uncle before the rest of my body does.