I used to exercise to get better, and as for me, the ability I seem able to improve physically (as far as cardio endurance goes) hrough training is very limited. I saw some studies a few years ago that indicated I wasn't the only one, and about 10% of people in a well controlled study saw very limited improvement through training.

However, the mental benefits are key. So the reframing matters to me. If the purpose of cardiovascular exercise was just to run a marathon, it'd be a waste.