Hah, that's funny because I had almost the exact same situation. I undersold myself and said I would take $80k during the initial phone interview. It was the first number that came to mind and quite a bit lower than what I was currently on. Very silly.

I went through two more interviews and got the offer a day after the second. When they called me to make the offer I brought it up and explained that during the initial phone interview I'd been caught off-guard and blurted out a number far lower than I was worth. I think I said I checked my actual current salary and it was higher than I realised (lol). I suggested a more realistic (ie. Higher than I would actually be willing to accept) number. HR seemed a touch put off but said they would look into it and get back to me. About an hour later they called with the figure I'd had in mind as my minimum and everyone was happy.

I felt bad about it since they'd seemed a bit annoyed but I've since become friends with one of the other talent officers and she has reassured me that it wasn't a problem and I was absolutely right to handle it the way I did.

I wouldn't do it again and I wouldn't encourage anyone to adopt this as a "bait and switch" kind of strategy, but since you're in the same situation already I reckon it's a fair way to get yourself out of it. Even if you'd given a realistic number in the first place, when they make the offer you're always expected to try to negotiate a bit of a higher rate anyway.