Start out in a low gear and maintain a good spin to save your knees. Frequently get out of the saddle to stretch your back out and when you do so shift up one or two gears in order to more effectively use your gearing. Velcro-type shoes are great so you can change the pressure on your feet while riding. Insoles or orthotics also help greatly for avoiding hot foot and general foot discomfort. Staying comfortable will do wonders for your peace of mind, enjoyment, and efficiency.
Pacing is a skill that takes many miles to develop.
Ideally, a double should not consist of one five hour and one seven hour century, but two six hour centuries, for this will be infinitely more enjoyable and comfortable. Try to keep the following rules in mind. Don't ride with a paceline which is faster than your ability. Optimize your rest stop time. Pay attention to your current and average speed.