Eat something beforehand that you know you can tolerate. I need some fat and protein in there, as well as carbs. Even on an easy (for me) and flat 24-mile ride, I find that if I don't have that on board, I'm hungry by mile 10-13.
I would also take something along--peanut M&Ms are supposed to be good, as are dried fruit, nuts, whatever you can tolerate while exercising. (I bring Shot Blocks or Clif bars.) Just in case, you know?

I've never really needed to warm up during a ride, but that's just me. It depends on your fitness level and the group you're riding with.

Have fun!