I'm not disagreeing with the above comments though I am curious about the factor of how much energy output a person is actually capable of. Running isn't leisur

I'm not disagreeing with the above comments though I am curious about the factor of how much energy output a person is actually capable of. Running isn't leisurely and you can reach (as far as I know) maximum physical (cardiac?) output running. My point being it's not like there is unused capacity to work harder when you're running that's making you slower. So there must be more to it than just doing more at times you'd otherwise be idle.