Trust me, you will definately not be the only person showing up clippless. Heck, I see people showing up on Orcas just to ride 25 miles. Charity rides are not races, but you'll find people who show up and pretend to race and you might even learn some things on a charity ride. My best advice about the carity rides is for you to stay on your guard at all times because there are always a few first timers or people not used to riding in packs. For the first 10 miles (or until the first rest stop), I would be extremely careful about who you ride next to.
I love doing charity rides, I have learned soo many new routes thanks to these rides. Enjoy!