I just completed my HM. I started very early in the cycle as my training plan was based on a solid 3 mile starting base. Thus I built the beginnings of endurance by running a solid month of 3 miles jogs at the frequency the training plan would have me running (minimum of 3x per week but there may be a 4th run/cross train event too). I would also try and run around the same time as your race (usually in the morning timeframe).
Other general rules I got from my father (while not Coach Nancy he is an ultra marathoner and another person I trust for advice):
Arrive well rested and healthy.
Go slow for your first 3 miles, speed up a little for your next 3 if your body is feeling good, your next 3 should be faster yet depending on how your body feels. It is all about the body as others have told you.
Don't drink unless you are thirsty. Also be careful of over hydration. A lot of people including myself were suffering from that at my HM.
Have fun. As this is your first you have no PR to try and beat so the goal should be just to finish. Don't stress yourself on a finishing time unless you were trying to use this to qualify for something else. Didn't sound like that from your post though.
I hope you enjoy your journey to the finish line. Good luck!
SEP12: Rock N Roll HM: 3:04:04
| current weight: 150.2