Just 80 lbs? That is a non-issue. One of my best students exceeded that easily when he started - he is slowly loosing it. I know a few others in the ballpark that once they got used to the exercise are having a ball.
Hope you don't mind me taking your post a little further.
Many overweight people use their weight as an excuse not to exercise creating as you might realize already, a vicious circle. I admire anyone who, other people's opinions be dammed, decides to do something about it. I'm pretty sure it took you years to put on the weight so don't expect it to come peeling off but Aikido (or any other exercise) is a lot more effective than diet (also important) in the long run.
As for your potential teachers and classmates, there is a lot of satisfaction in seeing their fellows improve both physically and technically. You will contribute to the dojo.
Last bit of advice. Push it a little but not too much. Listen to your body when it says to back off a bit.