First thing to remember:

The very important rule: 

If the race is important to you, nothing new on race day! 

Do not be tempted to try that snack new bar in the vendor village. You have trained hard for this event so no point screwing up your chances by putting something in your system that may not agree with you, or not digest properly leaving you feeling sluggish, or worse, with digestive issues. 

Breakfast

You will need to be prepared to consume a few hundred calories in the morning to ensure that you start the day right and end up properly fuelled to begin the event. 
*This may be hard for some athletes, so be sure to practice prior to the event. 

Breakfast should be easy on the stomach so it needs to be low in fibre and easily digested.  Stick with simple carbohydrates again like a bagel or even rice, possibly some fruit and just a little bit of easy to digest protein like egg or nut butter.

Include your normal coffee/caffeine routine.

In an ideal world you would finish breakfast 2 hours before race start and once you have eaten you should aim to consume 500ml of water every hour until you are out on course. 

Race Start
At the 10 minute countdown to race start, you should be drinking 1/2 to full bottle of sports drink or a sports gel plus water for a final carbohydrate boost. Do not take the gel without water as it will not digest well. 

Avoid taking in this fuel in the 30-60 minute pre-start window as you will lose the benefit of the boost and are more likely to suffer a drop in blood sugar at race start.

Front Half of the Race
In the “front half” of the race (especially the early stages) your gut is able to absorb the nutrients much easier than when your body begins to shut down blood flow completely to the gut in the closing stages of the race. 

In the closing stages your body will be sending all blood flow to your working skeletal muscles, which means limited flow will be sent to the gut and this often results in those dreaded GI issues. 

A common mistake people make is going too hard in the early stages and then failing to hydrate and refuel efficiently. This leads to a big drop off in performance thanks to the combination of fatigue, dehydration and low energy. 

General Rule of Thumb
Endurance athletes should aim to eat 60-90 grams of carbohydrate per hour during their event. You will need to practice and experiment to see what exact levels work for you. 

Fluid Intake
Your fluid intake plan needs to take into account the hourly amount of fluid you need (example, some athletes need about 1 litre/hour to match their sweat rate – others need half that amount and some need nearly double that).
*The only accurate way to really know is to perform a sweat rate test which means taking pre and post body weight checks when racing in similar conditions and intensity as you have on race day

Ensure you have practiced with your chosen electrolyte fluids and that it has enough of the critical component; sodium 800 mg/litre of fluid is a good “starting” amount. If racing in particular hot conditions this may need to be closer to 1000-1,500mg/litre while some people suffice with only 400-500mg/litre,

Back Half of the Race
The total fuel intake here is typically lower. However, you still need to ensure you are maintaining hydration and electrolyte balance (again – the importance of sodium) and going with more fluid energy intake, rather than eating bars/chews. 

Consuming a caffeine liquid, such as a flat coke on the last couple of laps is common place in endurance sports such as Ironman and can be beneficial but I generally wouldn’t recommend starting to drink coke until you’re at least half way through the back half. Ideally you would, if needed, consume something like this in the last 10K to 10 miles.

There Three Big Mistakes

1. Not hydrating early enough. If you wait to start your hydration plan this can result in early dehydration. This can have a huge negative impact on your race as your body then has a reduced ability to process fluids and fuel for the rest of the day. 
2. Not having enough sodium in your fluids. This means you do not ingest enough sodium which at best can cause bloating and at worst, potentially dangerous hyponatremia (low blood sodium) which can result in coma and even death. 
3. Taking on too much fuel and/or taking on too much fuel in one go. If the body needs to work at digesting food it will divert blood flow away from the working muscles to the digestive system and your race will suffer.