My neck and shoulders were so cramped that my accuracy started to suffer
One of the most neglected parts of the Fundamentals of Marksmanship is "Relaxation".
If you are getting cramped neck and shoulders, you're not relaxed. Age has nothing to do with it. I'm 65 and I have no problems getting a relaxed position.
When you shoot from the bench, you don't (or shouldn't) use your muscels to support the rifle. You rest your elbows on the table. But not the elbows per se, but the soft part of your forearms. Your head is relaxed, your resting it on the stock not using neck muscles to hold the rifle up.
Prone shooting is no different. Your head should be straight, bring the rifle to the head, not the head to the rifle. The head, or cheek is resting on the rifle where you create a "chipmunk" effect.
There are two types of prone shooting. Position shooting or non-supported (excluding the sling) and precision shooting, using a bi-pod or sandbags.
Both are the same in the sense you should be relaxed.
In position or sling, non-supported. Assuming the shooter is right handed, if left handed reverse this. Line up to the target. Your right knee is cocked and the rifle is lined up where if you drew a line from the muzzle it would go straight back to your right leg and foot.
Your left elbow is as close to directly under the rifle as possible. The sling is high upon the left arm, extended to the front sling swivel. The left hand IS NOT holding the rifle up. Thats the slings job. Your left hand is slid between the stock and sling. The hand does not grip the stock, but the stock lightly rest on the palm of the hand.
When getting an natural point of aim, you adjust elevation by sliding the hand toward the front or rear of the stock depending if you want to raise or lower the muzzle.
The butt stock should fit the shoulder at a point it allows the head to remain erect, and the head, or cheek should rest on the stock of the rifle.
You do not get a heavy strong grip with the shooting hand. The rifle should be supported by the sling and shoulder. The shooting hand only grips the pistol grip hard enough to keep your hand from falling from the rifle.
A strong firm grip with the shooting hand will cause you to disturb the rifle when squeezing the trigger. You should be able to drop your shooting hand from the stock to change magazines, load another round or write in your data book, WITHOUT disturbing your natural point of aim.
Set up your gear so you don't disturb you NPA doing the above. Set up your spotting scope so you can see your target by turning the head. You shouldn't have to move any other part of your body to look through the scope.
The left leg should be straight at an angle to the target to allow for a NPA, the right knee should be cocked, and high as comfortable. (In rapid fire, I sometimes get my knee right behind my right elbow to keep the elbow from slipping during recoil.) This method also takes the pressure off the diaphragm (which is important for us older guys getting soft in the mid section}.
I mentioned using the left hand to adjust elevation. You shift your whole body pivoting on the hips or middle to adjust windage. Once you get your NPA you should be able to close your eyes, take a deep breath, let it out, open your eyes to find out you are perfectly lined up with the target. If not, shift your body or move your left hand to adjust, do not mussel the rifle.
Close your eye, breath again, and check your natural point of aim. When you have it. Close your eyes, dry fire, open your eyes to make sure you're still lined up. Adjust accordingly.
This method will allow you to relax and prevent cramping and sore necks. If you cramp or get sore anywhere, you're not relaxed, your muscling something and your shooting will suffer.