You don't necessarily need to lessen the mileage, just take more breaks. If you want to do 8 hours of riding, take a half hour break after 3 hours, and an hour break after the next three hours, or something like that. Besides, you'll make plenty of miles in the flatlands, especially if you're going west to east. Even if you make a point of taking your hands off the bar and giving them a shake every few miles it will help immensely.