Set the plan to end on race day and miss out the first few weeks of the plan.
Riding outside on your SS is better than indoors on a gym bike for most workouts. If you are racing the 12 hour on your SS spend more time training on it than on a gym bike. You can do most of the workouts SS. The recovery and cadence focused workouts are the good ones to do on the gym bike. The interval and endurance workouts are best done outside on the road or trails.
Lynda