Practice is the single best way to nail down your nutritional strategy for a long endurance event. The more you practice your nutrition the more comfortable and confident you will become in the process.
It is important to practice what you will take in nutritionally on a regular basis so there are no surprises on race day. Practicing trains and teaches your body to process and absorb calories while you are at effort. Any problems you may be experiencing with the nutritional side of things can be worked out in your training sessions so there are no surprises on race day.
Practicing your race day nutrition should start when you are within 12 -16 weeks of your goal race it is a good idea to practice your nutrition on a weekly basis. The smartest time to do this is during your race specific workouts that target the pace you will be moving at during the event. It is important to train your body to take in and absorb your nutrition at the same effort you will be competing at.
Practicing race day nutrition is also very important on your long training days because the training itself will be easier to complete and recover from if you are consuming calories with regularity throughout.