This is something I never thought about when I first started playing as a youngster but lately its become an issue. Like many Americans I spend most of my time sitting - aside from intentional exercise and the daily requirements of living. Because of this I have a lot of back issues, I assume that is the reason any how. I have found specifically though after a lot of time playing my back kills me. Typically Mon-Thu I might log 3 to 4 hours of practice time. Usually on the weekends it could be up to 10 - 12 hours of practice time. Lately, on the weekends my back really starts to bother me, and it has become a big limiting factor on how much I practice. Usually its my back that tells me its time to wrap it up for the day. If I'm just playing through stuff I already know I'm able to keep a good posture but when I'm learning new things and deliberately practicing I find it very difficult to see what I'm doing without having an absolutely terrible posture. Any tips?
By "see" do you literally mean see (your hand(s))? If so, the guitar body angle has a bug impact here. But the ideal is to be able to visualise what you're playing (in your mind's eye), and that in itself takes practise (e.g. playing chord shapes with eyes shut, chord progressions with eyes shut, intervals, melodies etc) This isn't about absolute positioning on the guitar (eyes shut), but experiencing the feeling of moving relatively between shapes.

You could also try using a mirror, and stand or sit in front of it. Do you use a strap?
Typically I only use a strap when I'm standing. And most of the time if I am actually practicing I will be sitting. And by "see" I just mean if I sit perfectly upright I can obviously see the fret markers on the top of the fretboard but its hard to see exactly what I'm doing on the higher strings towards the bottom.
Obvious question ... but do you have back support on chair? Do you have a friend that can knows guitar playing and posture issues that can watch you? Or can you make a video ... maybe something shows there. Are you taking frequent breaks? (every 30 mins or so). As I said, the ideal is not needing to see the frets.