Less obvious but still interesting: At which point does diminishing returns set in? At some point does one extra hour of exercise yield less than one hour of increased average lifespan, and so not worth the additional effort, for someone that doesn't find exercise to be intrinsically rewarding?
Not at all. The only question is whether number of steps is a valid measure of how much exercise you're getting. Higher step count means people are walking more which means more general activity which is good. However, there are other relevant factors to consider such as walking speed, cardiac frequency, the intensity of the exercise. There are also other ways to exercise which don't involve steps at all. Weight lifting for example.
