I can walk from here to the ends of the earth, but thanks to a heart condition I can't run much further than 50 meters before I'm doing myself a real hazard of collapsing and ending up in the emergency room. Pushing my heart rate up is extremely hazardous to my health.
Cryin' shame, really, because in my younger days I could run a 5:09 mile. Not world class, but better than most. Until my ticker said "no mas".