There is a solid rider in my neck of the woods/desert who rides with his Jack Russell Terrier. The terrier just tears it up. I never saw a dog run so hard for so long.

The runners out here take dogs quite a bit. Some are not well behaved and cause some problems, CRYPT is right, make sure your dog will "follow your commands" when out on the trail.

IMHO a leash equals injury while riding a bike.