I forget where I heard it, but the best definition of mental toughness is that it's the ability to perform at or near your best regardless of conditions.
On a flat and firm XC course, two runners may run identical times. The next week on a hilly, muddy course they may be separated by 2-3 minutes. I see this all the time in high school competitions, and it's predictable once you know the kids.
The same thing happens in track. In perfect conditions two runners may have nearly identical PRs. Put them in a race with several additional stressors - rain, cold, wind, stiff competition, etc. - and you can usually predict who is going to have the better race. I attribute that to mental toughness.
I don't think mental toughness always determines a runner's PRs but it does have a lot to do with consistent success and competing well in high pressure situations.