Hi Dave!
I would also recommend salt tablets. Or something salty before. And drinking enough.
In regards to fat-adaptation, I wouldn’t eat too much protein, because that could also be gluconeogenic. I really like essential amino acids (EAA), while my long fasted runs. They are not insulinogenic and can even delay mental fatigue.
Are you using any caffeine?
For me, it helps, when I only use a little bit of sugar – like sucking on a gummy bear – when I start to feel sluggish, into my long runs. You get the carbs through your mucous membrane in the mouth and probably without any effect on fat burning.
Mahlzeit!