I have a six-year old male golden retriever. He is a little overweight, but not much ( I can feel his ribs if I hug him). Lately, when I walk him for more than a mile, he lays down and pants with his heart racing for at least an hour after the walk. Is this normal aging or could there be a serious medical issue? Should I stop walking him? I'm worried that if I do he will just get more out of shape. Help!