It doesn't sound like you said you'd pick her up, too, but obviously she has that impression, and you've let it sit there without speaking up, knowing that she is counting on you. How long ago was this conversation? If it's been a long time since the conversation, and the trip is fast approaching, I wouldn't let too much hang on what your exact words were at the time.
You were there, we weren't, and it sounds like you feel uneasy. It sounds like it might have been a while; you've given yourself time to think a lot about her other friends and relatives, the unlikelihood of her reciprocating, etc.
But I still think that you can get out of it. I would just apologize for not letting her know right away if it's been a long while and for the inconvenience of her having to make alternate plans at the last minute.
I STRONGLY agree with Toots that you do NOT try to offer alternate solutions -- i.e., don't suggest taxis, limos, long-term parking, etc. (she knows those things exist), or discuss why you can't do it, just say you can't. Definitely don't talk about her other friends or relatives, etc.
Another, in-the-middle possibility, IF you want to do it: tell her that 6:00 is just too early for you, but if they are willing to wait, you can pick them up at 9:00 am (or whatever suits you).