Staying in the aerodynamic position for long periods takes good core muscles. One thing to do is to go OTS every 30-45 minutes for a minute or so to give those muscle groups a break and to recharge. I practice staying aero during the winter when I spend a lot of time on the rollers and trainer by riding in the drops, tops' in the aero tuck, and on the hoods by rotating positions every 5 minutes.
Riding for 2 hrs at a hard pace will use up all the stored fuel from the blood, liver etc.. leaving only fuel from fat and what you are consuming while riding. If you are riding at 800-900 calories/hr you should be eating and drinking 400+ calories/hr. If you have used up the 2hrs of stored fuel and are not eating enough, all the energy that might be available is 300-350 calories/hr from fat. Continuously eating while riding, like the TDF riders, is one way of extending the stored fuel beyond the 2hrs allowing the body to burn more calories/hr. The processes are trainable over time.