Just say you changed your mind.
That is what I do. Then immediately leave. It is awkward for all of ten seconds and then your feel so much better.
If you stay you will feel bad about it all night, kicking yourself for not leaving.
Just walk.
If the woman is reasonably as advertised, so not a true bait and switch, just not your type for some reason, then pay a cancelation fee.
But I have no qualms walking if she is not as described.
I have also walked out over other issues. If a different girl answers and says the one I booked is busy, do I mind her, I walk even if she is pretty. If they ask me to wait for her to get ready I walk out. I walk out on principle as that is the only message I can send that clearly says "don't bs me".
With agencies and independents I deal with frequently that message has gotten through and they are now much more forthright with me on the phone about availability, age, description, etc.