The RAAM riders go everyday for 8-10 days straight, day and night and they don't bonk. Some of them do a liquid diet and eat nothing. They get their entire nutrition through liquids.
The biggest set of fatigue you will find yourself getting is mental fatigue. Watch the mind and how you are responding to things. That will tell you what to expect to come out of the body. When the mind starts to tire, for whatever reason, you will find the body will start to tire. Granted, this is also coming from the standpoint that you are in shape to begin with. Just remember the most important factor of life...the mind controls the body...not the other way around.