Might she be part Husky? They lloooooove to run, to the exclusion of just about anything else. You could be holding a steak, and the husky'd still opt for running, 99 times out of 100.

Pyrenees do tend to be very independent-minded at times, which is why they were such good sheep dogs.

On the occasions when our ould St, Bernard decide to run, she'd invariably come back mud-covered from the shoulders down, smelling of swamp. She loved chasing frogs! Wonder what the frogs thought?

I would just never let her off-leash unless you're in a fenced-in area. Annoying, maybe, but safer for her physical well-being, as well as your mental well-being. Any big fenced in parks in your area you could take her to every once in a while?