Main suggestion is to ride more. Your body will get used to the long duration of required effort. Right now it just doesn't know what to do.
Other tips:
1. Watch heartrate. High continuous heartrate fatiques the body much more than lower heartrate.
2. Stay hydrated. Make sure to drinkn lots.
3. Stretch when you stop for lunch.
It took me a good couple years to get used to long rides. Don't be discouraged and just enjoy the ride!