Originally Posted by
timmay8612
For the 12-15mi rides, I'm very comfortable the whole time provided I switch my hand positioning up here and there (mostly a hood rider). I went for a 20mi ride on fairly rough roads last week and the last 4 miles or so my traps, upper back, and neck muscles were killing me.
Sound like a conditioning issue to me. It might not make much sense drawing conclusions from just one ride.
You are asking your muscles to do 33-44% more work on the longer ride. Note that even 20 miles is a pretty-short bike ride.