I think as long as the dogs have been built up to this amount of running and you're keeping them well hydrated and watching for signs of distress, they should be fine. I don't know a lot about BCs, but as a breed they have been used to herd sheep over miles of land, to and from grazing pastures. They were meant for working, that's why they have so much energy, so I say if the dogs can keep up and are happy and healthy keep running. Since your dogs aren't herding sheep, this is the next best thing for them, they are very lucky dogs to have an active owner.