There are two ways to boost your writing output: you can work longer or your can work faster.
Instead of working for long hours at a sub-optimal productivity level, try the sprint technique: set a timer for 90 minutes and ruthlessly use that time in a "sprint" of writing.
After the 90 minutes are up, take 30 to 45 minute distraction break. Get up from the computer and do something completely unrelated to your writing. Go for a short run, eat a small meal, spend some time talking to your spouse or a friend.
Be sure your distraction break re-energizes rather than drains you. Try to avoid borderline-mindless activity that saps your focus, like surfing the web, checking email or gazing at your blog stats.
After your break, go back to the computer for another 90-minute writing sprint followed by another distraction break. Finally, repeat the process a third time.
You probably find you'll complete more work in three sprints than in an entire day of writing. And yet those three sprints took up just four and a half hours of actual work time.
If have the stamina, try including a fourth sprint and see how it goes.
This technique allows you to carve out relatively brief periods where you can protect yourself from distraction, followed by periods where you embrace distraction. I know of one author who used this technique to complete a book in one-fourth the time of previous books he had written.