I'll argue that "mental toughness" never, or almost never, exists in a vacuum. A person just isn't mentally tough for no reason other than they happen to be mentally tough. They got mentally tough by exposure to something that required them to adapt and they also have the ability to apply that toughness to other avenues. Maybe they worked a construction job, or their dad made them haul firewood as a kid, maybe they were on the wrestling team, grew up on a ranch.... there's all kinds of scenarios.
"Mental toughness", IMO, is the ability to compartmentalize discomfort and not get overwhelmed by the size of the task at hand, rather focus more on what is being done in the moment. Lack of "mental toughness" is "I can't keep doing this for 3 hours" where mental toughness is "I can take another step."
In hunting circles, mental toughness is often cited as something that can just be manifested out of nowhere -someone is just going to decide to be mentally tough for this one week of their life even though they've done nothing in life to prepare them for that and they have no frame of reference or experience to know what mental toughness even is. That's where I'm going to call BS. People who are truly "mentally tough" don't go around citing mentally toughness as the stand alone solution to everything in life because it is something they are constantly maintaining, further developing and demonstrating though voluntary discomfort whether they are consciously aware of that by always showing up and putting effort into their training on a consistent basis or they are entirely unware of it, possibly even resentfull of it, but they wake their ass up every day at 4 am to work a crappy, physical job that they hate because that is what is required of them.
I will add: never rely on mental toughness as a stand alone attribute in a hunting partner unless that aspect is proven. One of my partners had a knee injury 2 months before the season last year. He was unable to do any preparation and warned me that he was probably out. 10 days before the season, he decided he was going to go for it, but warned me that he was out of shape, slow and uncertain about his physical reliability. I wasn't the least bit concerned. Had that been an unproven partner, I would have told him to stay home.