I JUST shot my first
wedding this weekend. I really didn't really ready for it when I accepted it, and actually only did it because they were otherwise not going to have a photographer at all. I figured at the very least I could be better than no one at all. I told them that right up front, showed them my other portraits, explained the type of shot I was used to and just generally made sure they knew what to expect. ALSO during that time of communication, I made sure I thought they were the kind of people that I would be able to handle. I was ready at any second to pull the plug and tell them to look elsewhere if I got the impression they'd expect more than I felt confident I could deliver.
That's what I'd recommend for you as well. Being upfront and honest with your experience, and carefully judging their reactions to make sure THEY are the customers YOU want to have.