This is the single most important weapon in my success record. If you choose the liquid diet alternative, it is imperative that you have significant experience with the particular brand and usage well prior to the actual event to avoid possible digestion or nausea problems. Hydration is an equally vital component in maintaining constant energy and fighting fatigue. I always drink one bottle of plain water per hour, plus my liquid food. I know this is an awful lot of drinking, but it's a must.
Plain water is needed to break down all food, to flush the kidneys, and maintain hydration levels.
Staying comfortable during a double involves many factors. 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.