The best exercise for riding, is riding. The best way to not be sore is to get on sooner than later again. I can't think of any on the ground exercises that might help other than using a smaller sized exercise ball to work those inner thigh muscles and heel/toe pushes to get the calves.
I don't think I've ever "fallen" off a horse, I've been bucked off though, I consider that being removed unwilling, lol. Every time I've come off, it's never been my head. I've never "learned" how to fall off, we've always learned how to stay on! When working with young horses if you come off they quickly learn to keep doing it until it becomes of a problem if you don't. My last and worst fall was about 3 years ago, riding a supposedly "kid broke" horse and he just exploded, and it was a huge horse, I lost my stirrup and just when I thought I was going to ride it out he gave a huge buck and I was gone, all I remember was feeling the air from the hoof a couple inches from my face, and hitting the ground unable to breath. Cracked ribs all down one side, and a dislocated hip. It scared me, scared me good. I still ride, but it took some time to get back on, for the fact my horse wasn't broke and I was too **** sore, lol. Sometimes I still have problems with my ribs. My two horses are both going to get a couple weeks from my mom's guy they have train when they need it, both are started and calm, they just need work, then I'll finish them for the girls to use for 4-H.
What I'm saying, is I know things happen, and it's too early for me to be thinking.