The face looks great nice work so far.
You need to study up on neck muscles. The neck itself isn't too long but the shape and bulk of the muscles are causing the stretched look. Muscles don't really sag from one point to another what gives the neck its tapered slope is the build of different sets of muscles. Here are some basic land marks to get you started, you still should google up some neck anatomy and redo the neck.
The neck has a hallmark V muscle (called the sternocleidomastoid) this muscle attaches to the skull from behind/below the ears and runs to knobby center most tip of clavicle bones, actually behind it but the clavicle knob is a good land mark since it disappears there. You have this muscle modeled straight up and down.
Trapiezius, as far as front to back layers go, this is in the back row, it connects from the base of the skull (actually a little lower on the neck) and runs to the shoulder. As this muscle gets bigger it starts to create that triangle you see on muscle guys. On your model this is kind of a long slumping line.
Remember for muscles to work, they need to be like rubber bands, stretching and pulling but always kind of tight, never really limp and slack unless something is horribly broken. Again the build up of layers and groups of muscles is what gives the neck the slope from the shoulder to the ear.
Ahh hell heres a paint over...
Pay attention to the drop shadows, they give you some idea about what muscles are layered behind/underneath. Now go hit google, learn some muscle names, find out their shape and purpose and redo your turtle neck.