I second everything PaintedHorse said. (Have you seen any of his pics?? Totally jealous!! He's a trail expert!!)
I would add to his comment about horses getting sore, take Absorbine vet liniment and some gall salve, just in case.
If your horse is in shoes, make sure to take pair of boots just in case you lose a shoe! EasyBoot Glove is a great temp solution.
Here is a link if you haven't used them before: The Easyboot Glove | The Ultimate Horse Hoof Boot | EasyCare Inc.
I'll take a look at his photos.
My horse is barefoot, i'll make sure to check for stone bruises and i'll take a hoofpick on trail with me. He's a mustang with pretty sturdy feet.
Bring extra of everything. Extra halters, extra leads, extra hay, extra buckets.
Extra everything. Got it!
extra cinch, extra latigo, extra reins, duct tape. You might want to use a breast collar if you haven't before especially if you are doing lots of hills. Poncho (in case of rain). Sunscreen! Bug spray! Keep your cell phone on your body not in a saddle bag. If you get separated from your horse, he can't make a call. LOL
I'll be riding in my dressage saddle, I don't have a breast collar but I could probably get something like a jumping breastplate and use that with my saddle. I'd feel more secure if I knew his saddle wouldn't slip back when he gets sweaty.
I'll get a poncho. I need a cell phone holder that I can attach to me, but if phoenix could make phone calls that would be a real help.
Since this is your first time doing this and you do not know how your will react I suggest braiding some form of ID into your horse's mane. Name and contact information, etc. This way if your horse gets loose you can be contacted when someone finds him.
When I trail ride I have a big red dog tag that I braid into his mane. It has his name, my name and cell number etched onto it. My friends all laughed at me but I think it's cool. Plus it's shaped like a heart so it's cute and easy to see.