I will tell you the one thing I have learned about dressage - it's hard work!
I'm a little like you in that I pretty much just got on a horse and rode. I never had lessons, and have never, ever had any interest in dressage apart from watching the beauty of the horses.
So when I did a horse course a couple years back, I was thrown into the deep end. The focus was on dressage, so I had to go from riding however I wanted in a stock saddle to riding properly in an AP. I had to learn to leg yield, have mouth contact (a hard thing when you're used to using long reins), and look like a proper rider. I did okay, up until the time came to really get into the dressage side of things. I was pretty much dying after 5 minutes of posting (and I was getting ridiculously bored!). Didn't help that the two horses I rode, one of them was a headstrong Clydesdale x mare and the other a lazy QH gelding. The gelding had the same feelings as me towards dressage - I was doing a mock test and he sidestepped straight out of the arena and flat out refused to get back in!
I wish you luck though, as a fellow large rider
Having a horse that wants to and knows how to do the job should be a blessing.