What are endurance horses made of?

Are they made of the dreams of young girls, or old ladies who are just dreamers? It is certain that the literature available and the distance organizations all point toward the arabian horse as the benchmark if you want to do miles and more miles. A few weeks ago I was able to obtain a DVD that showed Potato Richardson on the Tevis trail, he was guiding the National Geographic crew alone the trails of the Sierra Nevada range. As those rides topped a mountain ridge, and the view spread out before them, my heart catches in my throat....yes, Endurance Granny wants to go there. While here in southeastern indiana the wind chill drops to -15 degrees, and I spend a half hour rounding up Phebe's the horse that would rather tough the elements than go into the stall for her evening feed of warm glop! Glop doesn't sound very romantic but it does warm and lubricate the horse's guts on these cold days. It is comprised of two pounds of beet pulp, one pound of Nutrena Safe Choice, and a couple gallons of very warm water. Phebes has been a problem filly from the very beginning. I worry that I will not have the skill or the means to make her the horse I believe she could be. Looking at her in her stall today (patched from a hole she kicked in it yesterday) she is blossoming as a coming three year old. No longer bay tobiano, now a rose grey, her head beginning to take on more of the arabian characteristics. Her chest is maturing and she is pushing 14.3 hands. Phebes does not have the conformation of today's show horse, but I do not see that as a negative. Her build is very balanced, and she has some powerful muscle. But she is willful, and very hot to handle. I struggle between my love for her and a deep seated frustration born of repeated failure with this very assertive and dominent filly. When the weather again turns to spring we will begin fresh, putting aside my feelings, and just working day by day to build trust and hopefully harmony. Any endurance minded readers out there....please do share your training methods, what has worked with your sensitive horse, I'm listening. ~Endurance Granny