Mastery does not come from how long you do something, but from how often. The quality of your writing does not depend on how many hours you devote to it as much as on how many occasions you give yourself to practice. Writing for eight hours on a particular day will not improve your style. On the contrary, it is likely to cause fatigue and injury, leaving your style weaker and clumsier than when you began. Writing for one hour, eight days in a row, by contrast, will build grace in your prose. Grace comes from being much stronger than a given task requires and strength comes, not from the exercise itself, but from the rest you get afterwards. It is because it is the regular alternation of activity and inactivity that develops your skill in any craft that repetition in training is so important. It is all about discipline, not endurance.