31 is a perfectly fine time, there will be others in after you, unless it's a very competitive race. Most 5Ks will have folks doing their first race and those folks might be walking and running. Besides, if you finish last, there's no where to go but up next time eh?

I agree with oakleaf, emily. All my race times are 1-2 minutes faster than I was training at, even though the percieved effort felt the same. It's the adrenalin, I think. And anyway, the last thing you want to do is finish a race and still have some left in the tank.