I'm only a beginner, (coming up to one year), but I've found that when starting, as I believe you are Norman, being strong can be a bit of a pain, because it encourages you to use your muscles rather than ki or whatever.
On the other hand, if you concentrate on not using your muscles when you begin, strength can't be that bad.
Being flexible is pretty useful.. makes you a good uke. You probably don't need to any specific training, since just the practice of aikido increases you flexibility, although I know more than one person who can't sit in seiza because it hurts too much...
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.