Originally Posted by Machka

Highly unlikely it is Shermer's Neck ... you're only riding 50-60 miles.
https://ultracycling.com/old/training/neck_pain.html

"Shermer's Neck typically starts between 300 and 1,000 miles into the race."


Could be posture.
Could be that you're unconsciously shrugging/hunching your shoulders when you ride.
Could be that you don't move your head much when you ride.
Could be dehydration.
Could be lack of electrolytes.

Could be ... and this I discovered recently ... that you've got the wrong helmet for you.