Have fun tonight, see you tomorrow!
I think no matter what you do with J, you're going to fuck it up.
I don't know even the basics of your non-sexual relationship with her, but here's my advice based on your responses above:
1) Be humble and self-deprecating. This is your fault, so act like it.
2) Don't try to use humor or wit to soften the blow. Breakups aren't funny.
3) Don't talk about the physical aspects of the relationship. It's irrelevant.
4) Lie, if necessary, to let her save face. If you REALLY care, you'll take the hit.
And, most importantly, don't ask for our advice and then not follow it. You do that a lot.