I would guess the idea of "mental stamina" means many things to many people. Certainly, the whole idea of having brevet series in increasing order of length goes to providing cyclists with opportunities to experience fatigue gradually.
I am guessing you are worried that you lose aerobic fitness rapidly and may struggle with a brevet series later on. I don't know what your your experience is but your organized approach to handling the physical aspects of brevet riding could easily supplant any need for much "mental toughness."
I think its the riders who make many mistakes early on in a brevet that end up having to make up their minds to over come physical limitations or other ride negatives. When I used to coach some newbies to new distances I simply told them - "You have to want it."