Great questions Katie! At least you are giving it a go. Did you attend the nutrition clinic at the clinic days? If so, we discussed a lot of this. I also do a deep dive in the Athlete’s Guide and I also just spoke through all of these specifics at the nutrition clinic that I lead at Peasantman. If you have missed all of these opportunities, let’s dive into it again…
You need 1.5-2.0 times your body weight in calories per hour on the bike. You need 1 times your body weight in calories per hour on the run. So you do need almost double the number of calories on the bike than the run.
If you are 150 pounds, you need between 250-300 calories per hour on the bike. As we talked about at the nutrition clinic, you want to eat 2-3 bites every 10-15 minutes. So eating every 30 minutes is likely NOT enough. If you just eat every 30 minutes and you eat too much at one time, you will end up with GI issues. Thus the reason why you should eat your 250+ calories as small bites on regular intervals. It is recommended that it be a combination of simple sugars and solid foods like PB pretzels nuggets or other salty, benign options.
As for hydration, you must be intaking ~1 bottle per hour. You cannot come back from dehydration so you can’t afford to get dehydrated. If you are a heavy sweater or need more hydration, 1 bottle is the minimum to start from.
So yes, the reason your brick runs are likely feeling so fatiguing is because you are fatigued due to low energy resources. You have your homework!