I think we may be refining the idea - especially with your clarification that you did not mean it to include things like macho, phony bravado, etc. (and I am starting to feel that I did draw an erroneous conclusion in an earlier post - I apologize for that.)

Might it be fair to say that mental toughness does come down to being slow to panic, willing to make the best of it when plans go awry, and refusing to give up when giving up seems the only thing possible? This would seem to allow the recreational weekend hiker to exhibit mental toughness right along with the AT thru-hiker (which would have to be, in itself, one definition of the term) or the relentless limit-pusher. It's the attitude, not the adventure, that defines it?

As an example, walking in the rain all day (especially when you really, really wanted the weekend hike to be dry) takes mental toughness. Getting lost in a corn maze and calling 911 is the antithesis of mental toughness?

It's been an interesting discussion.