Your main goal here is to be as comfortable as possible and complete the race with the satisfaction that you did everything right. Start the night before by making sure you are well hydrated before bed, using your urine color as a guide. It should be straw-colored. Avoid alcohol, which can cause dehydration and deplete your muscle glycogen stores.
Have a carbohydrate-based evening meal. This could be pasta, rice, potato or cereal based. Some lean protein is fine to include, but try and avoid anything too high in fat such as cheese sauces, pastry, and cream which can sit heavy and be slow to transit through your gut.
Eat your breakfast 2-3 hours before the race start. Again this should be carbohydrate-based, ideally with some high glycemic carbs included. Good examples include cereal, porridge and jam/honey, toast and jam, or cereal bars with yogurt and fruit. This is to help top up your muscle glycogen levels, which is the stored energy in your muscles and liver.
You should also aim to drink at least 500ml of fluid from waking to the start of the race. A drink with sodium in it will be better retained by the body; electrolyte tabs like SiS GO Hydro are an option for this.