Did you notice we never got details where he walked the second 500 miles?
He may walked in one direction. Then he found out she lives next to him and walked the whole 500 miles back!
His goal was "just to be the man who walked a thousand miles to fall down at your door" so it's not impossible he just walked 2000 laps around the block she lives on.
Fun fact: all the imperial units are now fixed on the SI system. Meaning that their formal definitions use SI units. So it's literally just SI with extra steps.