My target last year was to run a 5k in under 19 mins. I was running just under 21 mins at the time. The year was going great and managed to get to 19:30 at around June/July and was pretty confident of getting under 19 mins. Then all over a sudden I just burnt out in August and could not get back in to it. It totally ruined the rest of the year and the confidence which I had, was just gone. So I am back to it this year, but am concentrating on higher miles with lots more easy and recovery runs. I have my first 5k race at the end of march (have not raced at all since I burnt out) so will see where I am at then.
Also, I found sub 20 was a mind thing. I knew my body could do it, but my mind was holding me back as I was not confident enough at the time.