It is so important to do what you have practiced. With that in mind having a carb and protein based breakfast would be ideal and something to practise before your long runs before the race. My suggestion would be to have a oat milk porridge with protein powder 2-3 hours before the race (base the amount on 1g per kg of BW of carbs and 30-40g’s of protein) and then 45-30 minutes prior have another small hit of carbohydrates – this could be a few jelly babies for example.
There are so many ways to train for a marathon and many people may have experimented with training in a low carb state to promote better utilisation of fat when running. If you have tried this approach then that is great and has been shown to be really effective, however research still shows that although training in lower carb states can be really beneficial for training adaptations, performing in a high carb state gives the best performance outcomes and is important for maximising available energy on race day.
Another thing you may want to consider is consuming some carbohydrates during the run. As the marathon lasts over 3 hours for most people this can really help keep glucose levels up and provide the body with a needed supply of energy. An electrolyte drink made up of approx 2:1 glucose-fructose would be ideal and having this at the rate of around 90ml per hour. To be less complicated a lucozade sports drink sipped throughout would be sufficient. Again if you want to try this you need to practice drinking during a training run, please don’t decide to so it on the day or get tempted by drinks that are being given out if you have never trained with them…no one achieved a good time when they ended up in the portal loo all race!