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.