I have also become NO FAN of organized rides due to too many people coming out dressed for a Western Horseman photo shoot with the common sense and riding ability of Daffy Duck.
That said, as a lifetime trail rider and someone who is used to 20-30 mile rides:
1. Tell your co-riders even though your horse is supposed to be a seasoned, well mannered trail horse, she is new to you. Therefore would they all keep a reasonable distance from her butt, in case she is a kicker.
A lot of horses have no tolerance for another horse riding up on their butt plus is rude and lack-of-common-sense riding on the part of that rider.
If you need to pass another horse & rider, holler "coming thru!" And they should have the courtesy & common sense to move over.
Notice I used the phrase "common sense" a lot
2. Most long hour rides have stops at creeks for the horses to drink. Let your horse drink from a moving body of water, that you don't see a lot of scum buildup, like ponds tend to get.
When I lived in SoCal's Low Desert and rode the rock hills, several hours at a time, I taught my horses to drink from sport water bottles.
2.1. I might also pack a small grooming brush to brush the horse down on the days they work up a sweat. Maybe a couple of paper towels to wipe the sweat of the nether areas.
3. I always brought a hay bag to tie at the trailer. That is the ONLY time my horses ever needed hay. It was more like gum chewing for them, they didn't really need the food.
3.1. Along with the hay bag, I carried water from home (I had a 30 gallon water tank I had built for myself) and two water pails; one for the horse to drink water, the other to put a capful of Absorbine in a five gallon bucket of water and wipe the sweaty yukkies off my horse, especially legs, shoulders, hips. To me, that is more of a job-well-done reward than food.
If it was warm enough when I got home, I hosed the horse down anyway and cut him loose without feeding him. They do NOT need grained as soon as they get home, hay is ok, feed pan feed is not, IMHO.
4. If I am gone long enough to pack a NON-SPOILING sandwich for myself, or the ride sponsor is providing a lunch, I will carry a zip bag of chopped carrots and apples for my horse - not because the horse needs something, just to make me feel better, lol