I just finished my first charity ride last weekend, about 65miles also. I felt the same way as you...hadn't been on my bike in two weeks (traveling), and the ride was 15 miles longer and a 5 mile climb more than I'd ever done. I was scared, but kept reminding myself that there would be lots of folks in front of me, and quite a few behind me too.
It's really dry here in Colorado, so I drank a TON of water and electrolytes and therefore had to pee at EVERY stop. That was annoying. But I made sure I ate and drank frequently (shot blocks, gels, powerbars, fruit...something). That was a big help. So make sure you don't let yourself get hungry!!!
It's okay if you're in the slow group like me (I'm pretty quick on the flats but hills kick my arse)...just set yourself a goal. Maybe that goal is simply to finish; maybe it's to finish under a certain time. And set mini goals as you ride. At about mile 52 I was getting pretty fatigued, so kept saying to myself "I'm going to make it to the next intersection" or "next rest stop".
And enjoy the scenery. Living in the city, as I do, it was so nice to be out in the country where I could hear crickets and smell cows (yes, it's a good smell when you're a country girl at heart and see hawks overhead. Besides, it takes your mind off your body, if only for a moment.
You'll be great! And others will encourage you along the way.
Finishing my ride was one of the most gratifying experiences ever, but I'd also made up my mind to be okay with it if i didn't. We all know that some days, no matter what our training level, we're just not on our game. So make peace with that too. Takes the pressure off and lets you enjoy the moment.
Have a great time!! And like me, I'm sure you'll be jonesing for the next opportunity .