If she's going to run away, I don't see how I would be able to stop that anyway.
I would hope I would do a good enough job raising her and that she would have enough respect and admiration for me, that the option of running away would be far and away in her mind. Let alone wanting to get married that early. It's possible she falls in love with an older man at a young age, but with good parenting, I don't think she would have any issues playing things out.
I remember trying to run away many times as a kid...the farthest I ever made it was to the driveway. Chalking that up to "Hey, I have it pretty frickin' good here, even though mom won't let me do xyz". Most kids that run away are fleeing a bad parental relationship, could be abuse or just pure negligence and lack of love.
So again, it comes back to good parenting.