I think that if you want to systematically train for aerobic fitness, you're better off not counting on aikido to do it.
LOL What I'm in the same federation you are, I thought?
I lost almost 45 pounds in the first year and a half of Aikido training.
I find my aerobic endurance is better than my fiance, and he's a runner.(I HATE running
Aikido is my only source of exercise. I've kept myself at a healthy body weight, and have toned up more doing it.
other than a slipped disc in my back, I'm in pretty tip top shape.
I never turn down some one who asks me to take ukemi, and I try my best to take good ukemi. The side effect of that is a pretty good cardio-health.
Soft ukemi wears you out quicker, because it engages more movement(though is easier on the joints at least for me), I usually sway to soft ukemi when I'm looking to sweat. It is a challenge to get up as quickly as standard ukemi from soft ukemi, so it can really get you going.
Our federation, with Shihan who are such amazing teachers of Ukemi, there is no reason not to be pouring with sweat after every class.
If you have time to do more than Aikido for aerobics that's great too. I just don't have time for anything else and have found I'm in a good spot regardless.
*stops her sales pitch*