Yes, that's the problem with a lot of practice .... the diminishing returns, where you are just spinning your wheels. Sometimes, maybe a lot of the time, it's not easy to tell, especially when there's no teacher in the picture. How can you tell when you're falling into that "mindless" area??? I mean, when you're there, how are you supposed to be present enough to tell yourself that you're just goofing off???
The best thing that ever happened to my guitar playing (which I've been doing since age 15) was taking up the trumpet. It's such a difficult instrument that I had to develop a much better level of concentration just to deal with it. That spilled over to my guitar playing, which suddenly took off.
I'm feeling like, to get to the next level, I have to develop more concentration. More time practicing, by itself, won't do it. How can a person learn to develop even more concentration, when there's not a specific hurdle to get over to make it happen???