Some writers can work anywhere, in any circumstances, with any implements, on a schedule that only their muse is herself fully comprehending. The rest of us need to set goals.
Take Graham Greene. According to legend, he wrote 500 words a day, no more and no less. Take this recollection from writer and editor Michael Korda, who was introduced to Greene while cruising on a private yacht in the Antibes in 1950 (as one does):
An early riser, [Greene] appeared on deck at first light, found a seat in the shade of an awning, and took from his pocket a small black leather notebook and a black fountain pen, the top of which he unscrewed carefully. Slowly, word by word, without crossing out anything, and in neat, square handwriting, the letters so tiny and cramped that it looked as if he were attempting to write the Lord’s Prayer on the head of a pin, Graham wrote, over the next hour or so, exactly five hundred words. He counted each word according to some arcane system of his own, and then screwed the cap back onto his pen, stood up and stretched, and, turning to me, said, “That’s it, then. Shall we have breakfast?”
By the way, the novel Greene was finishing in such an offhanded way was The End of the Affair.
In any case, the lesson from Greene is a good one. Once he hit his word count, he supposedly quit and went off to live his life; possibly one of the reasons his fiction is so richly imagined and deeply reported.