I talked to a friend who is a 100 mile racer.

He told me the following:

It’s all in your head. No matter how well you prepared.
When running a half you bonk at 10.
When running a full you breeze past 10 and bonk at 20.
When running a 50 you breeze past 20 and bonk at 40.
When you run 75 you breeze past 40 and bonk at 60.
When you run 100 you breeze past 60 and bonk at 80.

I wouldn’t know about anything beyond the half.