IMHO Dave, a half-hour session is about all you can expect of yourself, especially if you are working intensely on new techniques or trying to conquer a rough spot.
The key to accumulating the many hours needed is NOT a brute struggle to stay in the chair long beyond the point where you're not progressing. It is to have the persistence to keep coming back for the next half-hour session (and you can do three or four a day if you can figure out how to space them) -- even when the last session sucked and made your fingers hurt and the dog howl.
What "keeps me practicing" usually is knowing what I sound like when I don't.
Cheers and good luck
-Glenn