Just go. I was an out of shape 41 yr old, just cleared for return to normal activity following abdominal surgery, first time I bowed in. That was 18 years ago this month!
Well Happy Birthday Janet
Well I have had people in superb running shape start Aikido with me who had real trouble in the first few weeks and one guy at 160 Kg who had his own problems. Both adjusted and did well. OK the runner did better faster but so what.
Point being the best way to get in shape for any activity is to do that activity. With Aikido especially physical requirement is really defined by the individual and practice can be adjusted accordingly.
And yes 'I am not in enough shape' is a classic excuse not to get in shape (start a physical activity).