When I was a professional riding instructor 8 years ago, I could not tell you how many people I encountered that thought you just got on the horse's back and the horse did all the work. I guess they watched too many movies and the actors made it look easy.
I cannot stress enough that horseback riding is an athletic sport and as such the rider needs to be physically capable of riding. That means they have to workout as much as the horse does to be ridden. You cannot get on a horse with injuries like a bad back or swollen wrist and expect to handle a 1000 pound animal and be safe. Also you cannot be obese and try and ride, yes I have encountered that scenario more than once.
Because I rode with my students I also had to be in better shape than they were. There was one time that I rode out on trail with a student and the horse I was riding decided to scrape my leg on a gate as we passed through it. My knee hurt like hell for the remainder of the ride but it would have hurt more if I walked. Back at the horse trailer I had my first aid kit which included ace bandages and a genutrain knee brace. Thankfully someone else drove us back and I put ice on the knee. I had to cancel all my lessons and rides for the next week until my knee healed. The trail accident was not a traumatic one like getting thrown from a horse but goes to illustrate how easy it is to get injured while riding.
So if you are considering to embark on learning to ride or are wanting to get back to riding again after a long period of not riding – get into shape first.