You need to train your whole body, not just upper or lower. I recommend running 2 or 3 times a week for a good half hour or more on top of your normal aikido training. And as someone said earlier, skipping rope is great too, especially to warm up. That should cut down on the "sluggish" feeling quite a bit. Weight training helps too but the consensus has always seemed to be that bulking up is bad for aikido due to loss of flexibiliy/mobility, so stick to lower weight and higher reps and make sure to stretch and warm up/cool down properly. And again, as someone said earlier, dancing is great for aikido. Being good at one generally makes you better at the other. So far it's worked great for me.