The body has about two hours of carbohydrate fuel (glycogen) at marathon pace (defined loosely as 85 percent of your max heart rate). Beyond that is the dreaded bonk. To simplify the complex science on bonking: body go boom. - Trail runner magazine, David Roche
Interesting article about fasted training. In the past I would always take food with me whenever I went out for over an hour but in recent weeks I’ve been doing these runs fasted and they’ve ended up just fine.