For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
"Walter Mitty" does not fit the three-act plot breakdown. Act I ends when the hero is fully committed to his journey, yet Walter Mitty never fully commits to any journey. You could argue that his "journey" is his imaginings, but the story begins and ends with a man already committed to fantasy instead of reality. There is no discernible "journey" to be found here, as this is not a grail-quest-type story.