Introducing the concept of exceeding one's limits is an interesting twist to the concept of mental toughness. In that perspective, "mental toughness" might fuzz over into one definition of insanity: doing the same thing over and over, expecting different results.

Or, perhaps one aspect of mental toughness is having sufficient objectivity to realize that things change, and the old solutions you came up with no longer work.

I found, about 10 years ago, that the body wasn't quite as resilient as it used to be. Joints were talking to me more often, feet and leg cramps became companions after long days, etc. Since there's a limit to how much conditioning can offset these effects of aging (and a limit as to how much time there is for conditioning in an otherwise full life of spouse, job, grandkids, etc.) I had to change my old ways.

Although I really liked the gear I was carrying, I could no longer haul 30 pound loads around anymore. So, I made the switch to lighter gear and new techniques that took me down to a 20-pound weekend load. That helped with joints and trail-weariness (and I've still got the ultralight option, when I need to get to a lighter pack in another 10 years.) It required me to rethink how I cooked, and how roomy a tent I needed, and several other things - it required a mental shift from "cushy" camping to minimal-without-discomfort camping.

It also required accepting certain things: given the practical limits of my ability to do conditioning, I had to give up 15 mile days, and accept the fact that in easy terrain, 10 would be my limit - and more like 8 if there were a lot of up-and-down. Foot and leg cramps were a 15-minute expectation at those limits - but working through them was a whole lot better than not backpacking.

So, perhaps one part of mental toughness is the ability to accept and adapt to what you cannot control. (Now, if I could just learn to accept those day-long rain hikes...)