Follow everyone's advice to do some riding (take it SERIOUSLY from a horse person!)beforehand. Otherwise, no matter what kind of shape you are in, you WILL have thighs that will be screaming at you (constantly).
The horses that most outfitters have are what we call "easy keepers".........which is just a polite term meaning that they are almost as broad as they are long!!!! Just joking of course, but seriously, you won't see many "skinny" horses on an outfitter's string. To a non-horse rider, it is difficult to explain the "stresses" that a horse's back will place on your inner thighs......it is something that will be eventually discovered (following a multi-hour ride over rough country).
No matter what shape you are in, if your legs are screaming at you, it will make walking excrutiating (even on flat ground).
My advice.....find someone with a horse that will let you ride (often) or sign up to take lessons (at least you will get some saddle time).