It is adjustment that is the problem. When I first went road I had a similar problem with the neck but due to the position and not a medical problem. Took a while but I spent parts of the ride adjusting position to ease the neck and also kept moving the neck around tso it did not get set in one position. I suggest you do shorter more frequent rides and hope the neck will adjust-But also take time to e "Exercise" the neck in those rides.
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How long was I in the army? Five foot seven.
Spike Milligan