To "run a 5k" means to take a shit, or dump, or some form of fecal expulsion.
It is in reference to one Warren Buchanan, who, whilst running a 5k for charity, was prompted at mile two and a half to run off the course in a nearby parking lot and pop a squat. Also keep in mind he was wearing an all-white track suit at the time, which is why he had to run off of the course immediately, and not run the additional seven minutes to finish the course in fear of defiling himself in front of hundreds of runners.
"Hey Bill I gotta go run a 5k, that bean burrito ain't sittin' right"
"Hey Tim I can't talk now, I'm running a 5k...could be awhile"