did you ever have any problems with their joint's?
people who run solely on tarmac quite often get bad knees or ankles because of the repetitive movement wearing down the exact same spot of cartilage,but then again a dog's weight is more distributed over 4 legs than our 2.
Ideally i like to run them on hilly uneven grass,or at very least like was said get a good mix of surfaces.
dont no about the dogs but its killing my feet lol