I run ~50 miles per week and have done maybe 15 marathons.

Drinking sufficiently becomes increasingly important when you go above ~10 miles in a race. Eating as well. You have to get used to it.

I recently ran a 50 mile trail race in 8.5 hours. Then I drank in total 6 liters of sports drink and ate 2 gels per hour, along with some candy bars. This is a pretty standard amount.

Pro marathon runners don't have to eat much, because they spend so little time out anyway. The slower you are, the more you will have to drink and eat