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.