Max runs with me everyday! I started him slowly. We would walk and then run for 5-10 minutes over certain areas of our morning walk. He quickly learned where his run sections were and would start pulling like crazy to get there quicker. We increased over time and he can now run 3 miles easily.
Three things to look out for: Be prepared to stop quickly. I don't run a fast pace with him b/c he gets distracted and has to stop occasionally to sniff.
Another thing is to keep an eye on are the pads of your baby's feet. I take Max on the trails as much as possible. Lastly, make sure your baby stays hydrated and watch out for the heat.