Since my marathons have all been German and French IVV marathons, I rely on the food and drink supplied at controls. Tea every 5k Along about the middle, light sandwiches, cake, and apple and orange slices are usually supplied. I will also have a schnapps, for the pain in my feet. I used to have a beer, which is probably better for dehydration but, if there are not a lot of bushes, causes other problems. Water, more water, lemonade, coffee with milk and sugar when you feel wobbly.
Some people prefer cola when their stomach gets upset.
Whole grain stuff is also too much when you do a 50 km. Have easy to digest food. Whatever you like. Remember, YOU wanted to do a 40 or 50 km. So stop whining and eat and drink.