I think Annie Dillard is speaking about the discipline of writing here, and perhaps about perseverance and yet she never says either word.

“A work in progress quickly becomes feral. It reverts to a wild state overnight. It is barely domesticated, a mustang on which you one day fastened a halter, but which now you cannot catch. … You must visit it every day and reassert your mastery over it. If you skip a day, you are, quite rightly, afraid to open the door to its room.”

from “The Writing Life,” 1989 hardcover edition, page 52