Make sure you eat a balanced carbohydrate-rich diet the night before, as it is often difficult to eat enough breakfast early in the morning. The longer the duration of the planned ski tour, the more important it is to have full glycogen stores and a balanced fluid and electrolyte balance.
A regular supply of fluids, carbohydrates and electrolytes is crucial for sustained performance over several hours. Drink regularly and according to your thirst. Sports drinks, ideally prepared warm, are ideal for this. For further energy supply, solid food is suitable as a snack in between.