Hi Amy. I totally hear where you're coming from and understand your fear, having just gone through a sort of self-exposure this week on a much smaller scale. A couple of things I learned:
1) I didn't have to share any more than I was ready to
2) The exposure was SO MUCH BIGGER from my perspective than from others'. I expected a harsh response, but I got nothing but love and support. In retrospect, I can't even remember why I expected such judgment
3) Being honest was freeing. I had held this particular secret so close for so long that I had forgotten to notice how much it weighed. Obviously you are not going to share everything about yourself to this person on this occasion, but not hiding who you are is easier than putting up a wall. As we learn in acting class, "Always tell the truth; it's easier to remember." Which doesn't necessarily mean volunteering everything about everything on the first date.
I hope you can enjoy your picnic and hike just for what it is, a little bit of time spent outside in the company of someone you might want to get to know better. Hugs and best wishes to you.