The only way to make a good trail horse is to trail ride. Preparing at home only goes so far. Nothing is the same as actually getting her the experience.
Just go out, ride, and have fun.
Exactly. Many people think that trail riding is just a simple, lazy walk down a path that any horse can do without any effort at all. In reality, every ride is different, even if you are riding the same trail over and over. Horses do not like change, and out in the real world the smells, sounds, wind, animals, etc. are always different.
You can work on de-spooking at home with plastic bags, tree branches, etc, but keep in mind that your horse knows that she is still at home, and home is a safe place.
People often ask me how you get a horse like our youngest (7 years) to the point of our seasoned, 'bombproof', lead mare (17 years), and I always answer '10 years in age and a couple thousand miles'. There are no shortcuts...enjoy your mare and ride, ride, ride.
As for slowing her down, if she hasn't been worked a lot, very fresh horses will typically have that "let's go" attitude. First, never let your horse pick the speed, even if you feel comfortable going fast(er). If I'm on a fresh horse that wants to go, I'll work lots of up/down stop/walk/trot transitions and lots of turns to insure I'm controlling the speed and getting some softness.