"The Secret Life of Walter Mitty" is a very famous short story by James Thurber, first published in the year 1939. The story follows the titular character, Walter Mitty, as he goes through a very mundane day of driving and errands with his wife and daydreams about extraordinary situations in order to escape from his unsatisfying, humdrum life.
The story actually starts out directly in the middle of one of Walter's day dreams, where he imagines that he's a commander about to steer his "huge, hurtling eight-engined Navy hydroplane" through a hurricane, with his crew is cheering him on. As it turns out, in real life Walter is driving his car with his wife, Mrs. Mitty, in the passenger seat. It seems that Walter's imagination has had an impact on his driving, and Mrs. Mitty jars him out his daydream by yelling, "Not so fast! You're driving too fast!"