Have you ridden any distance before?
Has the bike setup been OK for longer rides.
Does the bike fit you in its general size and its fine tuning of the position...
There are several points you should look at.
Is too much weight distributed to the front, ie are you too aero.
Do you grip the bars. Is your wrist set neutral, or do you have a bend at the wrist.
Is your upper body relaxed at the shoulders, and do you ride with flexed elbows ?
Are you bars too wide (ie your arms triangulate out) , too narrow, too deep?.
Does your bike generate and transmit a lot of road buzz.