How should I train to think while tired?

[Thoroughly]

I am building my fitness in preparation to enlist on an 18x contract. A coach and former marine suggested that in addition to a physical workout regimen, I start preparing mentally as well, by trying to answer questions during the workout. I've seen similar exercises mentioned in descriptions of SEAL training (math questions while cold and shivering), so it seems like a good idea.

His example: Between sets he asked, "What is the cycle of operations of an M16A1 rifle?" "What is an O3 in the army?" "What is an E3 in the army?"
He suggested that I find a source for this sort of information and start studying it, and point him to it so that he will be able to quiz me on the information.

My questions are these:
-- Is this a good idea? This is information I should learn, but is it the sort of thing I should be using for this kind of mental training? If so where can I find a good source(s) to study, and what subjects should I include? Based on advice given in other threads, I assume I should include a substantial amount of military and SF history?
-- Is this a good way to go about training my mind to perform/adapt/solve problems while stressed and exhausted? If not, how would you suggest I do so?