I almost always ride alone--it's a great start that you know you can trust your horse. They have cell phone things that can attach to your leg or arm (don't know if yours ever falls out of your pocket or not) but I use those in the summer when I don't have pockets..and sometimes even if I do.
As far as dogs go, I usually turn around and face them. They usually will back down. If they continue, I walk my horse toward them, "chasing" them. IF you encounter a truly vicious dog that is biting/attacking your horse--take off. You can outrun them. I've never seen that happen, though. Every case I've run into the dog has backed down when a horse came after them if not just when the horse turned to look at them.
It's a good idea to bring a saddle bag with a horse first aid kit (at least some vet wrap and gauze) some water, and a snack for you....just in case you get lost or an injury occurs and you're stuck on the trail longer than you think you will be. Most of all, just relax and have fun! Riding alone is so peaceful :)
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.