Did my first 5K on this Saturday past!! It was full of hills, and I had to take an emergency break just after 1K (!!!) to pee.... ugh!!! So the "official" time was 43 min (and 1 second), but i figure if it wasn't for having to stop, I'd probably have been closer to 39/40 minutes... lesson learned!!
Another important lesson I'd like to share: don't arrive to a 5K an hour before. No point - you just end up getting cold, and wandering in circles for no reason because there's no place to sit down.