How has he been taught his leash work? Most of us are taught heel on leash at a normal pace, slow and fast paces (so we change it up and the dog has to learn to adjust). Is he comfortable walking on loose leash or does he want to be in charge there too? Maybe he's had too much freedom/ choice (which actually is great for developing joints, but not for what you are wanting here..). He may just not understand why you are running now, and insisting that he's on leash.