It's interesting how our bodies react differently to training. I ran pb's at every distance last year apart from 10km (which was because I hadn't raced one) and it all came off high mileage, for me, marathon training. It saw me run 5km pb's as well as marathon pb's!
I tend to find it's when I can do the high volume stuff regularly that my pace starts to pick up.
I quite enjoy a 5k. It's pure agony for the time you're doing it, but it's over quickly.