This was a very small 5k in Clayton, NC.  There were only about 60 runners.  I decided to race it because it was described as "flat."  Being from Wilson, NC, I know "flat," and this course was not "flat."  There were a number of sizable hills.  However, I still felt I could run my first sub 20 minute 5k.  

I ran the first mile like an idiot, especially the first quarter.  I ran the first quarter in 77 seconds, which was about 20 seconds too fast.  I ran the first mile in 6:09 (16 seconds too fast).  I felt my legs tightening up soon thereafter.  I was able to hold on and run two 6:40s and finish with a new PR of 19:56.  I know if I would have run a smarter race, I would have easily been around 19:40.  You live and you learn.