For your first double, try to take it easy on the ride. You may want to shoot for an average heart rate a few beat lower than for your centuries. If there are any hills, try not to attack them or stand while climbing. You'll want to conserve glycogen as best you can. use energy gel, or some other easy toconsume food on the bike. A gel pack every half hour plus sports drink will get you up to 250 cal/hr.
After you get the first double, you can work on faster speeds. But for the first, be conservative to avoid bonking.