You've only been on one date with her, even if it was a long one.
It's a shame because you clearly get on well, but physical attraction is important to many (it is to me as well).
If you're not physically attracted to her it's not exactly going to go very far.
Just be honest, but not brutal. Just tell her that you had a nice time but you're not ready for dating. Or you could say that although you had a nice time, you don't feel you clicked in THAT way. Don't tell her you want to be friends because it can lead to false hope, seen it happen too many times.

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