As for endurance, I can't really say due to my lack of randori experience. However, from what I've read of people's belt exams, everyone seems to be completely knackered after the randori.
Methinks that is caused by the stress of the moment not a lack of endurance. I watched a shodan randori where the guy did 5 (yes 5) ukes and controlled it perfectly. He wasn't tired. One might argue that he was more ready than most for his exam. Relaxation helps endurance a great deal.
On the other hand, if I had to vote for one area that Aikidoists would do well to work on, it would be endurance.