If you can get to the point where writing is like a faucet you can turn on at will, you will be able to get a lot accomplished. What I mean by this is that you should be able to sit down, open up the document, and work on it with good results. The problem comes with not being able to even open up the document, or thinking that the writing has to be equally "flowing" every single time.
I actually have this capacity. I can turn on the faucet and work. If I am able to open the document on my computer and begin, I will be productive for a few hours no matter what.
Now the mistake people make is thinking that every writing session will be equally good, or that the bad ones don't count. I've covered this before on SMT, but I have new readers I didn't have a year ago. My theory is that that bad days and the good days average out to steady progress. You might have a day when you get an exceptional amount of writing done, others that are good but not exceptional, still others that are below average for yourself, and still others that are even more frustratingly slow. All you really need, though, is enough average or below average days. The great writing sessions are just one end of the bell curve, just like the very worst ones at the other end of the curve.
From this perspective, it is a mistake to say to yourself you shouldn't even bother turning on the faucet today, because you are a little tired / distracted / whatever and won't be able to write very well. What happens then is that one end of the bell curve consists of days when nothing was done at all. The writer also misses those days in which she might gain energy in the very act of writing. She is eliminating many of those "average" days that make up the bulk of a large writing project like a dissertation or book.
Teaching, service, even family life, can be draining in that you are giving your energy to other people. After a lot of that, you might be too exhausted to do research, which is extremely hard work. I am lucky that I find replenishment in writing. If I am too tired to write and write anyway, then I feel less tired afterwards.