Should I let my mare gallop when lunging?

I usually start out with ground manners, by backing, disengaging hindquarters, desensitizing, then I start lunging. She will go around at a trot and change directions for me a few times and then I ask her to go again and she takes off and gallops as hard as she can. Should I let her run or what should I do to correct her? I'm afraid she is going to hurt herself or me. I usually stop her and spend more time desensitizing and when she is calm I try lunging again but she just takes off galloping. She is outside 24/7 so she can run whenever she wants, what if I get on and she takes off with me on her? I'm kind of new at this, she was trained to lunge when I got her, I've had her about a year now. She is only fed hay, no grain.