Hi
I've had problems with my neck for years, not running related (only just starting that!), but maybe I can help with some stretches. This is from seeing a physio about 18 months ago. In my case it's not cured it, but it helps me manage it. My problems are probably mainly postural/enviromental (still working on that!), may have started with some whiplash, and may be now developing into arthritis... Anyway...
Of course, I'd say see your doctor really... and get referred to a physio!
Start off imagining your head is on a board and slide it back and forward 15 times (sort of like the sand dance but front and back instead of side to side!)
Next turn your head side to side as if you were trying to look behind you: left then right alternately, 15 times. Turn as far as you can, but keep it slow.
Next put your head on your shoulder (or as close as you can get it), 15 times one side, then 15 the other.
Next is one I've had trouble with, but it's good if you get pain/stiffness where your neck joins the back of your skull. You can do it lying down or sitting anywhere, but it's easier to 'get' standing back against a wall I think. You want to push/slide the back of your head up slightly, but without tightening the muscles under your chin, and hold for a few seconds. Reapeat a few times.
I also do some others that involve resistance against a handhold, but I wouldn't like to try describing those! I think they could be dodgy if they were done wrong.
Other advice: never 'roll' your neck to stretch it. Apparently the up to date advice is that your neck is designed to move in one direction at a time, so rolling stresses it.
Look at what could be contributing in your enviroment: pc screen should be at eye level, pillow low etc.
Get a massage whenever possible. And work your shoulders too: a lot of tension in the neck can come from the shoulders.