Personally,i think Aikidokas don't participate in the UFC because, 1.Those who are skilled enough to win(old masters) aren't realli interested in it. 2.Those who are interested(young,un-wise) are not skilled enough to join.
I mean,if you were a highly skilled aikidoka,why would you want to join? Wouldn't it be better if you had spent the time training in your dojo? There's nothing to gain,nothing to prove,or should i say,there's no need to prove. You are just what you are,no more no less.
Regarding the subject on stamina to last in the fight,i hope some people from the ki-society could give their comments on that.