No fresh piercings. There is absolutely no way to keep any portion of your body sterile. A friend of my stepson found out the hard way.
Just have fun - and hydrate. If its your first one, you probably won't have a good enough starting position to be competitive, so don't get frustrated by the number of people struggling in front of you at the various obstacles/challenges. If you can get into the general atmosphere, it can be a pretty good party crowd (just unusually fit). I recommend finding some friends to run with - the more the merrier.