Hydration is definately the key. I race SERA enduros too, so I know that type of heat you are facing. Temps are routinely in the 90s with 80+% humidity. Heat indexes are well over 100 degrees :scream:
The real trick to not to hit the wall. Once you get there, you're toast. I've spent many events lying on the side of the trail cursing the event, the organizers, my club, my friends, my bike, my dad because I wasn't having any fun.
You're taking the right approach, the first thing you must do is finish and finish all the time. NO DNFs no matter what. (Of course, don't push yourself into heat exhaustion and have to go to the hospital either.) Be smart and don't push beyond your limit.
You must remain hydrated. I start drinking tons of water the week of the event too. I've also found that good diet helps. Eat lots of fruits, and Carb load the night before the race. Pasta is the favorite amongst most of the riders.
I found that I wasn't putting enough food in my body during race day. I usually have a bowl of cereal or two, and a banana or two before the race. I also drink a lot of water the morning of. This usually means having to pee on a tree just before the start. Once the event gets going though, and you start sweating, you won't have to pee anymore.
I get the power gel packs that runners use and put them in my fanny pack. I eat one at the first reset. Usually the club provides water, so I drink that and save the water in my camel bak.
Enduros aren't a fashion show, so take an old Jersey and cut off the cufs (Arms & neck). This will let air flow up the sleaves. I have the Moose Sahara jersey and it helps. On the really hot days, I keep towels in the ice water of the cooler, also bring a gallon of water to pour over your head and face. I'll even soak my jersey in the cooler water and put it back on soaking wet. You'll find that it'll be bone dry in about 6-8 miles.
I usually eat another banana or piece of fruit and a peanut butter and jelly sandwich and a snickers bar at the first gas stop. This will usually carry me through, and I won't be starving during the end of the race.
Hope this helps, it's not too scientific, but I've found that it works for me. I started scoring a lot more points when I started beating all of the quiters. From there, you can worry about actually racing guys in your class.
Good luck and come over and introduce yourself at the races. Look for me at the race in Louisiana on March 23rd. I'll be on row 45.
Oh, for training, start jogging in the heat, this will help with cardio development. Once your boddy is used to the conditions things will go much better.