Really great advice here from dardusm. You should practice with diligence for short bursts of highly focused disciplined exercises (you know why you're doing something, what you expect to gain from it, and have a way to measure your improvement). Then you need to "test" that improvement with some short competitive sessions. They don't have to be gambling (but can be), tournament play, or league play. All of those help, but you can compete against yourself (play the ghost), or play with your neighbor and get the same results.
Scott Lee
http://poolknowledge.com
Scott Lee
http://poolknowledge.com
The problem that I see with practicing for such a long stretch is the ability to maintain focus. I think you can do more harm than good if you just go through the motions without focus for hours upon hours. If you are matched up with someone, that focus is motivated by the match/money and could be much more beneficial. But, with that said, if a player has the motivation and focus to play for hours and deliberately practice, they are going to improve.