Quality over Quantity

I've talked to many people who say that they train for 10 hours a day every day or six hours without stopping. In fact I used to say that myself. I just recently read a book called The Art of a Beautiful Game. One of the chapters are called Training: Preparing like pros (and Feeling their Pain). It's about the author going to a training camp where the two trainers trains everyone as if they were NBA players. The trainers said in an interview with the author that they believe in Quality over Quantity. They said that if you are working hard enough, you cant go six hours straight. Now when I go to the courts to train or to the gym to lift weights, I ask myself, "How hard am I willing to work today? Will I work Hard for an hour or will i work at a 50% level and train for six hours?"