The hardest thing about writing is immersing yourself in it when your brain is fighting you.
The second hardest thing, after you've gotten into a habit or flow of writing, is coming up for air.
Once you've exhausted all the internet distractions, jumping up to do a load of laundry, and whatever else you do to avoid work, you find yourself writing and getting into it.
But how do you stop? I've been working in the mornings, trying to take a break in the afternoons to get class work done, then going back in the evenings, which is, whether I like it or not, the time when my best ideas come to me and the best writing gets done.
When you take that break, though, your brain doesn't know enough to say "Switch now! No more thinking about the manuscript. Put those ideas down and hop on the Comma Splice Express for some paper grading!"
With toddlers who have a tantrum when they have to put away their toys and take a nap, we say that they have trouble with "transitions." I guess academics can have that, too.