Re: Pretty disappointed in myself, looking to fix that.
There's nothing to feel ashamed of. The only thing to ever feel ashamed of is not acting. You wanted to train and workout at the same time, there is no shame in that. Aikido, it is true, is usually not an extravagant workout and will not trim you down - you should do some other exercises for that. But it'll help.
You have returned to us now and as you said, you know that you want to do Aikido. So do Aikido and enjoy it! Devote yourself to studying the art, because to know that you want to do aikido and to not act would be shameful - whereas your previous actions were not.
I'm sure what you probably feel worse about is having shown up to a class and now..having to go back to that dojo and say "heh..I was silly, I want to train here." But it isn't really necessary. I'm certain that most sensei's are used to people coming and going, and they aren't going to immediately latch onto you with hopes of your long-term commitment. Our dojo in particular (being on a university campus) has a low retention rate. Much like Tomiki, we don't expect most people to stay very long, and most don't.
So you don't need to apologize, just show up and dedicate yourself - and they will all be most happy to see you.