Our yellow lab is about 12 years old now, but ever since he was about 3-4, he has pretty much behaved as you described. Yes, he will run at times, but he would MUCH rather just sniff around for signs of other dogs (and of course raise his leg to mark that he was there). If I take him to a dog park, he basically just walks from smell to smell and checks everything out.........zero interest in running and playing with other dogs. That's one reason we don't go to dog parks anymore. I'm pretty sure my dog has no thyroid issues or other major health issues....it is just the way he is.