Miles miles miles. The more you ride her the less spooky she will be. Also I agree with above about ride with a purpose. If you are spooky and are watching for things that will spook her she is going to be spooky. They pick up on things like that. Also talk to her in a calm, low voice. When she starts to spook just say, " easy, whoa girl, it's alright." Let her know you aren't worried. If she sees you as the herd boss, which she should, then if you are calm and collected, she will pick up on that.
There is no sport like barrel racing. It requires more speed than a sprinter, more balance than a gymnast, more grace than a dancer, and a stronger team than football.