By no means a careful experiment, the difference between my 20k (12.4 miles) and half marathon (13.1 miles) was striking.

I ate two bananas before the 20k. I think the run started at 8 AM. I ate normally the day before. There were four aid stations, and I ate a Gu at the second and fourth stations. Around mile 10 I felt like crap and walked for a bit. I finished ok but really didn't feel great.

The half marathon started at 7:00. I got up around 4 AM and ate a Pro Bar Meal (390 calories), drank a bottle of Gatorade (160 calories) and had some instant coffee (150 calories). I took with me to the start a Stinger caramel wafer cookie (a knockoff stroopwafel and 160 calories) and ate it about 6:30. During the run I ate three Gus at miles 4, 6 and 9. I felt strong the entire run. At the end I felt like I could have kept going for another half. Three days after finishing I couldn't wait to do another and signed up for two more halfs and a full (November 2017).

Everyone is different and how we train and how our bodies react will vary significantly. But I'll definitely be eating a lot before my next half (August 20).