This does need an explanation. And
@Sanchez is on the money.
This is going to make it sound like I consider myself an Olympic-class athlete. I have no illusions of that. I'm just a middle-aged-guy with a strong (self-)competitive streak.
For 5Ks (really all races except Disney races), there is no such thing as a fun run for me. I'm usually trying to PR, and if not a PR, still have an aggressive time goal based on my condition, weather, course, etc.
Marathons and half-marathons, while physically challenging, have a mental/management aspect to them. You have 26 checkpoints and 4+ hours to make adjustments.
5Ks are almost all physical. A good 5K is a near death experience for me. I say that only half-kidding. I figure if they don't try to usher me to the medical tent, I left something on the course. Again, this makes me sound like some tough guy, trust me, I'm not. Stupid guy would be more accurate.
It's like the difference between an Indy race and a drag race.
I was afraid I lost the ability to find that sharp edge and ride it to the finish line.
I was pleased with the results, so maybe I haven't lost it.