If you haven't already you need to start getting yourself and your horse in shape. A 10 mile ride doesn't seem that far but if your only used to doing a short 2-3 mile ride your going to end up with a sore horse and a sore butt
Just start doing the 2-3 mile rides at least 3-4 times a week working up to as far as you can go by the ride date.
Sounds like your horse is used to riding with other horses so there shouldn't be a problem there. I'm sure the St. Jude ride is going to be a huge crowd with all different levels of riders so I woulnd't worry too much about them going to fast or not taking regular breaks. Wish I could go with ya
Sorry I hit enter before I was finished
A ride like this one may provide lunch etc. check out what is being provided before you plan for anything in the meals area. Our club always lets the riders know what is being provided in our newsletter about upcoming rides.
For sure take some water, I like to take hard candy (jolly ranchers) with me and some horsey cookies for my girl.
Take a camera, some sun block, chap stick, I carry a horsemans knife all the time so that has all my tool type stuff. They are handy and not very expensive.
As far as grooming, I always clean Vida up before we leave the house, then give her a quick brushing when we get to where were going.
I can't think of anything else, except be sure to have fun