Maybe not the worst runner ever, but certainly one of the most bothersome:
There was an old geezer who used to come out to some local club track workouts and jump in for about 200 to 400 meters at a time (even if the distances on the repeats were 1,000s or 1,600s), taking several minutes rest before hopping back in for another short distance. He would start the workout with the fastest group and, after being left 20 meters in arrears by the end of the first 200 of a 1,000, would gradually work his way down to the slowest recreational group, where (after plenty of rest and psyching himself up for the effort to come) he would jump in for their final 200 and blow them away - well, barely struggle home in front, anyway. He would probably amass about 800 to 1,600 meters worth of running for the day, while everyone else was getting in 5 or more miles worth of repeats.
This wouldn't be so bad if you thought the guy was just a slow but otherwise fairly serious runner who was out there doing the best he could. But he never really did any running apart from the once per week he'd show up at the track. He didn't know anything about the sport and would spend most of his "workout" time annoying people with stories about giving his truck an oil change or about the benefits of investing in pork bellies.
He was probably capable of a 29-something 5k and could sprint a wicked-fast 35 for 200 if really pressed, yet once, following a workout, he told the whole group (in reference to a sub-15:00 5k guy who was standing right there), "Yeah, he might be way in front of me in some of these races, but he knows I'll whup him if I'm there with 200 to go." Then he looked at the sub-15:00 guy and nodded as if in some fraternal bond of comrades in sweat.
What a character.