A little advice on starting as an older person:
A lot of riding instructors started riding when young. Many can't remember when they were not riding. That is nice, except they will then give well-meaning instructions that an older body won't do. Jogging, for example, tends to tighten muscles that need to be loose for good riding, and I brought 40 years of regular jogging to my first ride. Someone telling me to point my toes forward, heels under hip, loose leg draped on the horse while moving my lower back with the horse...she might as well tell me to pick my horse up and carry it around on MY shoulders!
So when you start riding, jump on the forum regularly and tell us what you're seeing, doing, and what problems you are encountering. Riding involves a lot of compromises, and folks here can tell you their experiences and what compromises they have had to make. This place has overweight riders, young riders, riders with back injuries, riders with arthritis, riders recovering from auto accidents - you name it, they are here. And most of them LOVE to share what they have learned, how they have overcome something or how they have needed to compromise one thing in order to do another.
Lots of folks love "http://www.amazon.com/Centered-Riding-Trafalgar-Square-Farm/dp/0312127340/ref=sr_1_1?s=books&ie=UTF8&qid=1355960814&sr=1-1&keywords=centered+riding+by+sally+swift". I hated it, but I'm pretty much alone on that.
My favorite is only available as a used book - http://www.amazon.com/Commonsense-Horsemanship-Vladimir-S-Littauer/dp/0668057912/ref=sr_1_1?s=books&ie=UTF8&qid=1355960777&sr=1-1 by VS Littauer. It is written from the perspective of teaching jumping. I don't jump. I don't use an English saddle any more. I love the book anyways. It is one of the few books that suggest learning to feel your horse's balance and learning to work with it. Too many books discuss "position", when riding is all balance and motion.
"http://www.amazon.com/How-Your-Horse-Wants-Ride/dp/0764570994/ref=sr_1_1?s=books&ie=UTF8&qid=1355960846&sr=1-1&keywords=how+your+horse+wants+you+to+ride" is a good book written clear enough for a beginner. It was one of the few that made sense to me when I was starting.
"http://www.amazon.com/Hunter-Seat-Equitation-George-Morris/dp/0385413688/ref=pd_sim_b_30" by George Morris is another one I liked. Yes, it is about jumping, but there is a lot there that applies to all riding, written by a genius.
I mostly ride western in an Australian-style saddle, but don't let folks lock you in to some discipline. A dressage rider can give good advice to a barrel racer, and vice-versa. And someone who is "just a trail rider" can give good ideas to someone who hates trails. Also, remember that you are ALWAYS training your horse, including your lesson horse
: Every rider IS a trainer -- every time you interact with a horse
And this might wet your appetite: JUST Trail Riding???