On a related note with the arm strength... I have noticed my shoulder getting sore when I run 3-4 miles hard. The front part of my shoulder is what hurts, it's contracting as I'm putting my left foot forward and my right arm forward. It really gets worn out. I thought it was a conditioning thing, but since my left arm doesn't feel the same way, I'm assuming it's a form problem. It's almost like I'm not relaxing enough and letting my arm twist all the way back as far as it should.
I am also getting a worn out stomach muscle on my right side towards the center that goes all the way from the pelvis up to my chest. I am assuming this is because of my mismatched legs though. My right leg is longer than the left. In order for my knee to strike at a halfway correct angle and not hurt, I have to purposely twist my hips a little and force my left leg down a little. I think this muscle in my abs is what is helping do that but I don't know. If I don't do that little trick with my hips, my right knee will be in so much pain after 1 mile that I could not go any further.
I've always been a short distance sprinter so this distance running stuff is pretty new, not sure what to do about these issues.