Upper body pain on neck, shoulder and arms when running distance is common with some that just started running. Our way of sitting, standing and even carrying loads on our back contribute to the discomfort we may have when running.
And when we headed out to run, we start to over correct our bad posture and consciously or subconsciously hold our shoulder close to our ears or pull the shoulder blades together to imitate a better posture. You then start to pull your arms closer to your body.
Or in some instances, you let your body slouch and that action collapse the shoulder while restricting your chest from expanding and your airway blocked - occasionally looking up only when there are photographers around ;). Some of us run looking down because it was how we been running. If you ever wondered how and why you had that "less than good" run, it is your posture.
To start with, you need to be loose. The more fluid you move, the less likely you will tense up. You can only be fluid if you learn to relax. And you can only relax if you don't try to squeeze out all the blood from your clenched fist or be so stiff your shoulder freezes over. By being more fluid, you will have less pain. The less pain you have, the longer you can run. Sound like a good deal?
Starting from your hand, imagine that you are holding an egg in your hand. By doing this, you will allow some slack and prevent any tension or stiffness running upwards to your neck and shoulder. Bend slightly at the elbow and keep the hand between the chest and waist. I personally recommend nearer to waist.
Next, allow your shoulder to move along with your arm swings. Think of it like you are doing a light jab forward and do not cross your arms as you swing it. Move your arm in-sync with your stride. Your arm swing helps to regulate your stride.
Now that is the basic of getting the upper body fluid. Now, you go and try it and hopefully, you run better and with lesser pain.