You will have the time of your life.
I raise and train quarter horses and harvested an elk in Colorado in '04. Too much fun. I am so excited for you and your father.
My advice would be simple. Make sure you're as prepared as possible for the altitude and relax when you are on the horse. I'm sure they'll be really broke and well-trained pack horses. Be thankful you didn't have to pack your whole elk out on your back with your guide - whew- that was an awesome experience in itself.
Enjoy yourself and best of luck to you!

Kimberly