5k
That would be like signing up for Kindergarten after getting your masters for someone with your drive and motivation. Just sign up for a half and be done with it.
5k is a whole different animal. You mostly have two types of people doing 5ks, those who just want to get out and do something physical while socializing and those who want to compete and enjoy agony. For those in the middle, the hassle of signing up, getting there and back, etc doesn't offer much reward for the investment IMO.
That said, a mock 5k isn't going to test you like a real one. Not many people are going to do a training run where you are on the verge of puking your guts out for 20 minutes give a take a few minutes. The motivation of having other people to race is huge in boosting performance.