a personal anecdote - quite a while after diagnosis with heart condition, i found cycling was something i could enjoyably do. as i got more and more into it, even without any all out efforts, ever, i got to a pretty solid level for long distance rides. i could hold 200+ true average watts uninterrupted for hours and hours at 110 BPM. i did unsupported centuries with a few bars in my pocket, pretty solid 3,500 climbs, etc.

at the same time, walking up stairs or a hill causes immediate shortness of breath, and running is out of the question. my legs are strong, i’m not overweight at all, but my heart doesn’t work correctly but can manage cycling. the bicycle is an amazing device!