Consider the title of the story. What is Walter Mitty’s “secret” life? Who is it a secret from?

In the Story, Walter Mitty's secret life would be his daydreams and his imagination, and he hides it from everyone around him. By his daydreams it looks like Walter Mitty wants to be a hero but he doesn't do things about it (this is what his daydreams are about). In the daydreams we can tell he wants to be a hero whether it be a war hero or hurricane fighter.

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"

what is the book idiot with the brocken tester