As mentioned, there's one hill right out of transition and at the start of lap 2. Pace it smartly. Everything else is pretty much flat, with a very slight roller near the turnaround at 3&9 miles.
As mentioned, heat and humidity are the concern. For the faster guys, traffic on lap 2 and aid station congestion is a serious problem. Be prepared to skip stations of walk...or some being out of pre filled gatoraide on Lap 2. The worst was the station at think mile 2&8 if I remember.
You just hit a some congestion of lapped traffic it seems as the field "inverts" as later waves of mid 5 hour athletes pass earlier 7 hour athletes.... at the same moment that low 4 hour athletes are coming around on lap 2.
TrainingBible Coaching
http://www.trainingbible.com