I think this because to be even a mediocre marathoner, one needs to exercise with about the same frequency as the control person. But one also needs to work diligently on a large peripheral system like flexibility and mobility, recovery routines, stricter diet controls such as counting calories, macros, ensuring proper vitamin/mineral uptake, strength work with lifting, etc.

Sure, a mild exerciser can be relatively healthy, but the real difference is that the marathoner has challenging goals to accompany the systems for building health; whereas the mild exerciser only has the systems and easily accomplishable or even no goals besides "being healthy".