My 5K run was this weekend and it went just simply amazing! I ran the race I wanted to run.
I ran the 5km distance only a couple times before doing the event, but I learned some valuable things while running these distances. Things that were useful for me (may or not be for you):
1) Don't carry water with you if they have a water station halfway through, if you can. You should be able to go 15 minutes without drinking if you're well hydrated to begin with and the extra weight isn't worth it in my opinion.
2) Don't start too fast or you'll be burned out before the end of the race. Take the first half at a comfortable pace. It's tempting to follow the flow of the race or a faster person who is in front of you especially when you have the energy to do so (short term) but listen to your body and don't overdo it! Keep it for the end!
3) Running faster when it went downhill and slowing down for uphill was something I wanted to do, to take advantage of the free energy of downhill, and to not kill myself during the uphill. I think it was worth doing. I took advantage of the downhill and it helped me pass quite a few people who were going more steady, BUT it still took more energy than it should and made me have to slow a little afterwards... I think I gained more than I lost.
4) Give it your all in the final 300-500m, if you have anything left to give. Let the crowd motivate you beyond what you'd normally be able to do! :)
The race I wanted to run was a difficult race where I gave it my all, but one where I gave the right amount of energy throughout so to be able to continue on until the very end and give it a sprint at near the finish line. I did just that. I finished with a time of 26:32 which is better than I even hoped for and it's motivating me to keep training to bring the time down some more. It's all for fun but I am motivated by goals so I'm keeping that in mind.