Three miles seems a bit much to start a over weight dog out on. Just like humans the dog needs to start out short to go long.
I did'nt pick the distance...the dog did. I just made sure to watch him that he did'nt over do it. When I do see him slowing down...we take a break. I alway's carry lot's of water with me for him. So after about a 15-20 minute water break...we head back down.