Since it takes roughly around 7-10 minutes to run a mile, to "run a mile" can be used for "pooping" instead of blatantly saying one is pooping.
One can also say "2 miles" for roughly 20 minutes of pooping and so on.
Guy 2: I think I'm going to go "run a mile". Be right back.
Guy 1: Alright, I just ran my 3 miles this morning.