I've always kind of "felt" my distances, without much extra work. I started jumping when I was 5 (I'm 14 now) and for as long as I can remember, jumping just kind of came naturally to me. I really sharpened up my ability to judge distances by counting down strides. I had my trainer do the "3...2...1...jump" thing a few times for me and it just clicked. Now I can tell when I'm 20 or 30 feet out from the jump whether it'll be long or short, or if I should push her out or pull her back really easily.
As Oxer said, you shouldn't lean back. I think you mean you sit deep in the saddle, which is perfectly okay. Just as long as you don't keep your weight behind the vertical, because then both you and the horse will get off balanced. I also ride in half seat, which I find it a lot easier than riding with your butt in the saddle, especially because I ride horses with very bouncy canters, and I don't want to slam on their backs. Great job getting your distances though. It'll only get better