At this stage, a lot of it is just tensing up your body because of nerves. I sispect, also, a lot of newer riders are afraid to move around; throwing your shoulder into a turn, moving your butt forward and back and side to side, etc.
Riding without a windshield, you want to lean forward some so the wind supports your body- otherwise you're a sail and pulling on the bars all the time. This position distributes weight through your thighs more, too.
OTOH, leaning too far forward (rae-replicas) can really take it out of your knees, shoulders, and wrists. (Racers adopt this position because it provides way less wind drag and much better control of the front end)
Cruisers need back support because sitting up straight is hard on your back. If you don't believe me, sit up real straight right now for a while- at your desk. A windshield helps the "pulling" part, but it's still hard on your back. Plus, this position has you sitting squarely on your tail bone.
Another huge factor to fatigue is wind noise (loud mufflers add to that noise). Many touring riders use ear protection of various sorts.
Conditioning -concentrating on upper body strength- will help.
On long rides, I like to add a kidney belt, too. They're not just for hardtails.
Finally- dehydration. You simply don't feel your bodies moisture literally getting sucked out of you. This makes you tired and less focused (and cranky).