Hi John,
I suggest you get this checked out! It sounds very similar to the chest/shoulder pains that Gari Williams would get during our long runs last summer when climbing up some of the longer hills. He would slow down and relax for a bit. Then we could continue on for a couple more hours. He would get stronger as the runs went on. Unfortunately, he was avoiding a serious problem. I certainly hope that is not the case with you. You certainly are fit; so was he. It was a lesson for all of us to pay attention to the signals the body provides.