I'll give a quick background to my endurance nutrition needs for those of you who don't know much about it. You body needs fuel. Obviously. The problem many endurance challenges throw at you is just the volume of calories you need to take in. On average it's around 6000kcal per day. Your stomach and digestive system often can't cope with that volume of food so you have to change what you eat and when you eat it. Also many people (including myself) struggle with chewing food when really fatigued so have to resort to liquid diets, which actually is a good thing as it gets into your body quicker.
My first advice is CUT OUT ALL SUGAR. Sugar a very unstable source of energy and can make you feel ill. Cut it out NOW. No gells with sugar, no Haribo's, no chocolate.
Anyway, you need to think about two things. Fuel and Recovery. You can fuel yourself all day but if you don't recover properly after you will crash the following day.
Lets talk about fuel first
Your body needs calories and they come in the form of Protein, Carbs and Fat. Because endurance challenges are long slow and steady, fat becomes your best energy source. Also fat has 9kcal per gram whereas Protein and Carbs are 4kcal/g. So straight away you can see you need less than half the volume of fat to get the same calories. The high fat diet does work once you train your body to burn fat efficiently. There were times when cycling in Croatia I would eat (while on the road) full fat cream and smoked salmon. Protein and Fat. No carbs. This was quite extreme and nowadays I still include carbs.
So on an average day in the water or on the bike I like to have 120g Protein, 700g carbs and 150g fat. If I can have more fat I'll try but it's surprisingly hard to get fat on the road. Chorizo, cream and butter are about all you can easily find with high fat content but eating butter on its own is damn near impossible. Peanut butter and nuts are very good too. Also it's better to eat little and often. I like to eat every 90 minutes if I can. Always eat before you need it.
The more exercise you do the worse an athlete you become. Think about it. Most people run the second half of the marathon slower than the first. It's only when you give yourself the right recovery that you get better. For me this is simple. Protein and Carbs. I have about 30g Protein and 150g Carbs in shake form after every session. Also the best recovery meal is Lasagne. Spaghetti Bolognese comes in a close second.
I have simplified what works for me. Nutrition is a HUGE are and everyone is different. Here a a few other things to think about.
Electrolytes - These are mainly Sodium and Potassium which in very simple terms help your muscles use energy and water. I can't describe it here without putting some of you to sleep but research it if it interests you. They are important. Sodium is normal table salt and you get potassium in bananas.
Water - You need to drink a lot of water. Work out your sweat loss rate. Weigh yourself naked. Go for a 1 hour hard run and try not go to the loo. Weigh yourself after and deduct any water you drank on the run and you'll know how much water you have lost through sweat. 1 litre of water weighs 1kg. Now you know how much water you need to drink on average. If it's hotter you'll need more. Roughly you'll be around 600ml per hour or thereabouts. I like to drink 1 cycle bottle each hour but thats just me. If you drink too much you'll need to pee a lot and if you are in a close race that's not ideal.
Keeping healthy - This is a hard one. It's easy to say "No I'm not eating that salad because it takes up half the space in my stomach and is only 200kcal" You still need vitamins, antioxidants and other foods to keep you healthy. Make sure you have your fair share of fruit and veg to keep your immune system strong.
So thats briefly what works for me. Try different things and CUT OUT SUGAR.