Set goals based on output, not input. "I will work for three hours" is a delusion; "I will type three double-spaced pages" is a goal. After you write three pages, do something else. Prepare for class, teach, go to meetings, whatever. If later in the day you feel like writing some more, great. But if you don't, then at least you wrote something.
That was something a friend in graduate school said he did--writing two double-spaced pages of dissertation a day--and I successfully copied it and continue to do so in some fashion (though not necessarily always two pages). Those two pages aren't necessarily good, and will be changed through editing, but they represent a concrete goal and then accomplishment.