James Thurber paints Walter Mitty as an "average Joe" to beat all average Joes. He is not particularly good looking. He is not a physically imposing figure. He isn't good at doing mechanical things, and he is often forgetful. He gets bossed around by his wife. He's not a great driver. In addition, people don't treat Walter with a great deal of respect. He gets laughed at a fair amount for his various blunders. He is not one of the cool kids, but deeply wishes that he could be. Echosmith would be so proud.
The characters he imagines himself as have the complete opposite characteristics that Walter has in real life. In Walter's "secret life" he is a powerful,decisive, and coordinated man. He is a person that other people look up to. He is a guy that fits in . . . at the top. For example when he imagines himself as a great surgeon or naval hydroplane pilot.