your pain is most likely being caused by overstressing your trapezius muscles.

the position demanded by riding a bicycle (head up, back leaned forward) is common in few endeavors, and puts a premium on the trapezius muscles.

many believe that the position, in the extreme used by professionals, or less extreme as that used by amateurs, demands uncommon flexibility, but looking at the position when turned 90 degrees clockwise, and so looks as though one were simply sitting up against a wall with their feet on the ground, looking upward, and calves and thighs defining a 90 degree angle, is attainable, pain free, by almost anyone for long periods of time and demands little flexibility above that that is common to all of us.

the stress on the trapezius muscles is caused by the tilt of the head upward, in an attempt to see where one is going. with added stress coming from the leveraged weight of the head. which is usually supported by its alignment with the backbone. the head is essentially hanging on the end of a stick, and it weighs alot. trying to hold it up makes it even more difficult and it must be supported almost entirely by the trapezius muscles, which are connected to the back of our skull. hence the "neck pain" when riding distances that we are not accustomed to.

do what nature wants you to do. stop doing what you are doing (riding) until it doesn't hurt anymore. then start again after a day or two until it does hurt. repeat as necessary...

trapezius muscle strength (no, you won't look like the Hulk) and endurance will occur naturally with repetition. it may take months or years depending on a combination of genetics and application.

watch the pro's. on the track and on the road. when they tire, their heads drop, and they end up looking at the ground. but not for long. it's too dangerous to ride like that.