...That's what I mean when I say without focus you can't accomplish anything.
After focus, the next most important thing for a novelist is, hands down, endurance. If you concentrate on writing three or four hours a day and feel tired after a week of this, you're not going to be able to write a long work. What's needed for a writer of fiction—at least one who hopes to write a novel—is the energy to focus every day for half a year, or a year, or two years. You can compare it to breathing.
In private correspondence the great mystery writer Raymond Chandler once confessed that even if he didn't write anything, he made sure he sat down at his desk every single day and concentrated. I understand the purpose of his doing this. This is the way Chandler gave himself the physical stamina a professional writer needs, quietly strengthening his willpower.