I was so pleased with myself and praised me more than any other boss would have done. I had a very self-satisfied smile on my face throughout the day and hope I shall have many more days like this. It is easier once you've passed the moment where you think you can't possibly continue.
It was good productive work too. Somehow my brain went into a creative overdrive and the ideas were bursting out. Perhaps all the pushing liberates a flow that doesn't get tapped at other times.
So, if you're tired and thinking up excuses not to write, push hard. Chain yourself mentally to the chair and don't stop until you absolutely have to because of other commitments.