Very good point, but having some kind of work technique, you can work for hours without getting tired or bored or anything...
I'm not saying it works for everyone, since I have lost some time wandering around when I first started practicing for several hours a day. But if you have a good teacher and discipline, you'll have to work a tremendous amount of time to have mastery over your instrument. You need work and time, and you need to consider all the points you have made. Put them all together, and you'll work more effeciently.