Actually, you might look into the ride info... they will probably have SAG stops with food and water, and maybe even a whole meal at the ride start/finish, so you may not really need to take food on the ride, unless you need some quick shot of some sort.

I agree that some road riding before hand would be a good idea. I ride an MUP for a few miles to access the roads I like and the roads are much rougher than the nicely paved MUP. Riding on roads will give you an idea about things like how much tire pressure you want to ride on to get a balance of efficiency and comfort, if your gloves will be sufficient for hand comfort (rougher pavement means more vibration that is transferred to your hands), etc.

Don't know anything about mirrors or lights, but you can probably get some good advice from your LBS.

Good luck and enjoy the ride!! I love organized rides!