My Jack is 9 years old now, so biking for him is over. He is a lab x with collie and hunterway. Really bright, easy to train, at his peak 30 miles no probs. you need a dog that aint gonna get tangled in other riders/runners legs and wheels. At the top of descents he would always hang back and run behind. At the top of climbs we would stop let everyone catch up, Jack would get excited and whine because he was eager to get running again. Would avoid running a dog hard until its 2years old and fully developed, joints are soft until 18months old.