This is an easy one. The fact that you even know letsrun exists means you can probably break 19, the fact that you are familiar enough with it to know this will be a funny topic means you are a regular poster, which means you ran a 14 min 5k and make $900k a year and have a smokin hot wife and 11 kids. However, you are now bowling a half respectable score, which means you have other hobbies and likely aren't running 120 mpw, it also means that you likely drink too much, get regular second hand smoke, eat garbage, have low testosterone, and a messed up thyroid. You are a purist though, so you don't get any medical help for these conditions. This all tells me that you are probably getting a little fat, which you can't support because you haven't lifted a single weight in your entire life. I also suspect that you are currently fighting off a bacterial infection from the rental bowling shoes, further degrading performance.
All things considered, I predict a 17:45. Good luck at your 5k marathon!