Carb load in the 24-48hrs before. A little fibre as possible, you will feel uncomfortably full. This works for me on big hard days. And 100+g/carbs/hr during. Caffeine tablets - 3g/KG bodyweight after about 90mins should kick in nicely in the last 90mins.
Test all this and any other new techniques before the event
Sticking to a fueling plan during an actual event can be challenging, and it’s worth reviewing after the race how much you actually consumed.
I recently had a good (for me) performance on a very hilly race, coming in around 5 hours. I avoided cramping, and finished fairly strong, with power in the final hour being comparable to what I did in the earlier hours.
I was pretty disciplined about fueling: water with Skratch mix from a hydration pack, and two gels per hour. I targeted 90g/h of carbs but also about 600 mg/h sodium. The latter was a new point of focus for me, and I do attribute some of the improved performance to it. I also hydrated prior to the race using Precision Hydration PH1500 (1 tab in 500mL water the evening before; repeat 1.5 hours pre-race).
FWIW, for the gels I used one NeverSecond c30 and one SiS Beta Fuel. The big difference is the SiS has very little sodium, whereas the N2 has 200 mg. With that and the Skratch (380mg per scoop, running about one scoop per hour) I was golden. And both gels legitimately taste good (to me).
Sleep; tighten your bolts; don’t go out too hard; remember to fuel. Good luck.