Mitch, I was a lot like you....rode horses in a pasture when I was a kid, bareback mostly.....my uncle had a horse we rode and so did several of my friends....
Jump forward a few years....
I'm 55 now and started riding about 4 years ago....at my wife's request.
We trail ride almost every weekend.
I'm probably not the most "correct" rider out there, and I really don't care. Just about everything I do with my horse I enjoy and really feel like the more formal and competitive it becomes the less fun I'd have.
I don't plan to win a gold medal at the Olympics or a Gold Buckle at the rodeo...... but beware....One thing you'll find is there is no shortage of opinionated experts......
So, no, you're not to old.....just enjoy the ride and don't worry about what everyone else thinks you should do. No need to take lessons, If you want to know how to do something...... just ask the experts......lol.
I am turning 43 this week and just started English lessons earlier this year. Never too late! I am so excited for my lessons each week and cannot wait until I am ready for a horse of my own. I will say that it is quite a workout for me and definitely not as easy as some make it look! Well worth it at any age.
Falling off always sucked. I am 55. I fell off several months ago from a dead gallop. It hurt, but I didn't break. Riding should help you get in better physical shape and stay in better shape. A lot more people die from sitting on the couch watching their arteries plug than get killed in horse riding accidents.
Got my first horse at age 40. You're not remotely too old. If anything, you will probably find you turn back into a 7 year old when you swing your leg over for the first time after all those years of drought. Welcome back to riding. Don't worry about falling off. Just keep the horse between yourself and the ground. If that doesn't work, wine and a hot bath also work well.
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 :
I'm 48. I rode a pony once when I was a kid. Grew up in a fairly rural area around horses. Been around them most of my life but didn't ride. About a year and a half ago a good family friend who rode horses her whole life, trained others, was the person who everyone took their psycho horses to fix was to the point of giving up on life because of a bunch of unfortunate things that had piled on her including losing a very special horse that meant everything to her and was her primary lesson horse. I bought a similar horse to get her going and started taking lessons. Found out I have very good instincts about horses and probably the worst "seat" you'll find. I'm lucky if I don't fall off while mounting-it's that bad. BUT, they have changed my life, and if I could make a living raising horses or anything in the business I would-it's that good. It's never too late to start and you only live once. Churchill was right, the best thing for the inside of a man is the outside of a horse.
As to the size thing, just find a big enough horse, you might consider a draft or draft cross. If you like quarter horses there are some out there that will handle you easily. If you like gaited, find an old style big Tennessee walker. I'm 5'8" and 225 regardless of my current waist size. My partners 15 hand 1100 pound Paso Fino staggers every time I get on him and after he gets used to my weight he still stops when he gets tired of carrying me. My 14 hand 950 pound Peruvian doesn't even notice me on her back and will ride me into the ground. Find you a good settled middle aged horse who has been there done that and you'll do fine.
You're 47. Most excellent. You're humble enough to listen to your instructor. You're wise enough to realize that the horse is 1100 pounds and you're 140 or so, and you can do the math and have respect for the odds. You're young enough to be able to get the muscle-memory imprinted. You're experienced enough to keep you and your horse out of danger.
I didn't read all the other responses because I am a little short on time so if I repeat things already said I apologize.
In my opinion you are never to late to start riding, good for you for wanting to do it the right way taking lessons and learning before jumping in and buying a horse right off the bat. Not only do I think it will be great motivation for you to lose the last few pounds you want to lose it will definitely help you get there!
In regards to english or western, I ride both and enjoy both equally as much, one is not better than the other. I would dabble a little in both until you decide which one YOU enjoy more. They are both different but enjoyable in their own ways and you will probably find you will lean toward one more than the other.
Good for you for getting involved with the best hobby on the planet and we are looking forward to hearing about your journey!