Let me attempt a semi-formal definition:

"mental toughness"is that quality of perseverance and the ability to determine and focus upon matters of critical importance when the situation and conditions become adverse.

In other words, you do not become unglued when unexpected problems arise. You never know when your need for MT will occur; it might even happen on an anticipated easy weekend hike (been there, done that).

If you can deal with unexpected adversity, it probably follows that you can deliberately put yourself into a tough situation.

You never outgrow your need for MT!

If you develop some MT while in the outdoors, it may well transfer over to other aspects of your life- or vice versa

Lori, an interesting point of difference. Nearly all of the successful and capable SAR types I have worked with were outdoor types - fairly strong hikers and backpackers, cavers,or rock climbers, all with a wide variety of professional backgrounds - interesting difference