On those days when you feel unproductive:
Consider that you may feel unproductive while actually being highly productive. Samuel Johnson, the author of the first dictionary of the English language, referred to himself as a "castle of indolence."
Work on something - anything - however small, so at the end of the day you can note one thing that was done.
Recognize that being unproductive can be productive. You may be subconsciously preparing yourself to be more effective when you eventually turn to the central task. You may also be refraining from rash action that would be harmful. As Secretary of State George Shultz put it, "Don't just do something, stand there."
We were not made to be speed-reading minute-fillers. We were designed to be distracted and to ponder; to mess around and approach problems from different angles. Few lives and careers proceed in a straight line; instead, they resemble mountain climbers who occasionally move sideways in order to gain the better grip or clearer perspective that permits them to proceed upward.
In short, there are days when "being unproductive" is the most productive thing you can do.