My big advice is this(feel free to ignore it if you don't agree):
I think ANYONE who wants to ride endurance should do CTRs FIRST. Lots of them. They really prepare you for the distance in a more gradual way, and they teach you the ins and outs, down to the very last detail, about how to care for your horse. People who go straight to endurance sometimes overlook very important aspects of the sport that involve the horse's well-being and health. In a CTR I'd say any horse is fine. Don't worry about her conformation YET, you're only starting out and for this your horse will be FINE as long as you know how far to push her and when to stop. CTRs(competitive trail rides) are much more detailed in the vet checks and really make sure that the riders have proper fitting tack, that the horses are being hydrated properly, etc.
Anyway, here are some general tips, much of which you may know, and some of them apply more to CTRs than endurance, but it all connects:
--Electrolytes are commonly used to prevent dehydration (baby food carrots are really good to mix them with!)
--If you want to be competitive it's a good idea to have something on you (like a GPS) that will tell you how many miles you've made it so far...and also a watch so you can see how much time you've used up!
--Carry a sponge with you to sponge down at water stops, and REALLY sponge at the hold before your P+Rs are taken. Sponge really really well, try to get the whole bucket of water on your horse. (but not by dumping it)
--don't let your horse eat or move during P+Rs as it will make them go up...DO let your horse eat before gut sounds are checked!
--It REALLY helps to have someone experienced compete beside you your first time so that they can walk you through all this
--as far as for the rider: pack water and protein snacks in your saddle bag so that you make it through the ride too!
...that's all I can think of off the top of my head right now...any specific questions?
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.