There are a few pretty big mistakes people make while training. I find the following to be one of the worst ;
Going out too fast in the beginning of the run. The thought process ( If I run faster in the beginning, I'll go further, and that will compensate for when I get slower the second half. I'll run further/faster than the people who go slower and steadier or run/walk.)
WRONG!!!!!! WRONG!!!!! WRONG!!!!!!!!!!!!!!!!!!!!!!!!!!!
If you go too fast in the beginning you will deplete yourself and thus your second half will be WAY slower than your first. Those people pacing themselves will PASS you as you chug a lug. Also the people who went a tiny bit slower than you in the beginning will have way MORE energy for the second half and be FASTER than their first half.
Think of the Tortoise and the Hare story. Only in this scenario the hare has nothing left in the gas tank.
So how do you know how fast to go??????
When you train, do at least one run a week where you go out for a distance and turn around and come back. At the turn around, check your watch. Your second half should be faster than your first. Simple.
There is nothing worse than DYING at the end of a run and having everyone pass you, and no better feeling than WHIZZING past everyone at the end. Especially the ones that past you in the beginning and thought they were all that ;) In a short 5km, the people who went out too fast for their bodies, will get passed at about the 1.5km mark. Not fun.
Yes you can go TOO slow in the beginning, experience will help you with that. I use the breathing test. The first half of my run my breathing is up but I can talk to you. For my long slow runs I can talk to you the entire time, but the last few kms I stop. On a short run, I don't want to mutter a word the second half because my lungs are busy.
GO test yourself. With a little training you will know how fast you can start off, and the more races you do, the more you will know just how fast you can push yourself.