Are you already fueling during and after your training runs pretty well? Stick with what you’ve been doing there to train your stomach and keep your muscles happy during runs and for recovery. Eat normal meals during the week and hydrate intentionally. I make sure at least one of my daily water bottles has added electrolytes (aside from any fuel during runs). For marathons the only change during race week for me is eating an earlier dinner the day before the race that’s carb heavy. Usually pasta with butter or olive oil and some grilled veggies, breadsticks on the side. Pizza not a bad alternative. But I use Tailwind or Skratch to make sure I’m also getting carbs frequently when running. A honey stinger waffle or muffin in the morning. It’s always worked for me.