So, my father said that there were a few steep hills to go up. Apparently, (Maybe he was exaggerating, never know...) one of the hills was so hard to go up, his horse actually had to lift up from a canter to a gallop to get up it. This wouldn't be a big deal, except for Zena is a greenie, and I've never galloped on her.
This makes no sense. The horse does not have to move from a canter to a gallop to get up a hill. I used to ride up and down a ski hill for heaven's sake. No galloping there unless we asked for it. Even a canter -- now maybe the horse will need to lunge a bit if there is a short steep step, but it still should be at the speed that YOU determine.
1. If she were to start galloping up the hill, should I let her continue?
Don't let her start in the first place. Pay attention at the hills and tell your family (especially your Dad it sounds like), that you need to walk up the hills and please would they respect that. Since you already perceive this to be an issue, don't let it happen. Stop her at the bottom of the hill and maybe at the top of them too. Since you will be in the lead, you get to set the pace. :) would a one-rein stop just be way to dangerous?
Yup, as indicated above. Not on a hill. Learn the pulley rein as well as it doesn't require as much trail width.
2. What do you take with you on trails?
There are threads about this, but for the life of me I can't think of any search terms. Some ideas already above and remember keep your cell phone on YOU, not the horse. Doesn't do you any good if you're lying on the ground injured and your horse and cell phone have galloped off to Oz. Get your own saddle bags if this is going to be a new activity for you. It's nice to be able to reach around and get your bottle of water, or snack.
3. How often do you take breaks on a trail?
Three hours isn't long for a decently in-shape horse. However, it also depends on the pace you set and the temperature. Play it by ear and ask the other riders to help you keep an eye on you and your horse. I did a 25 km/15 mile ride one afternoon on somewhat of a whim, without any prepatory training. Oh, and it was after a bush ride of about 2 hours. I did about 80% walk and 20% trot. No cantering because it was riding on busy roadways. We stopped once for about 15 minutes because I needed to stretch my legs. My horse was tired but not exhausted and not sore after the ride. The next morning she was ready to go again. Given a choice, I wouldn't have done it, but the situation at the time was, well... unique...
4. Any tips, warnings, etc.?
Don't let the actions of the other riders influence your decisions if you feel iffy about something. You are only a novice trail rider, not a novice rider. And don't let your horse just do whatever the other horses do -- so often I see one rider go to a trot or a canter and the other horses will automatically do the same. They are herd animals after all. But I will ALWAYS hold my horse back and she is only allowed to catch up once she has settled down.