I have noticed that it's much easier to switch to fat-burning or ketosis when I'm doing endurance exercise that's relatively strenuous and lengthy (like hiking). I have no idea if I actually am in ketosis, but it does feel like I'm burning fat for energy. I get to a state where I am expending way more than I'm taking in but without any feeling of starvation. I have never had carb flu when I'm doing extended long-term endurance activities (a few days or more.) And if I do feel like I'm in a borderline carb flu state, endurance exercise seems to energize and restore me.
I sometimes read Barefoot Ted's forum and there are a lot of distance runners there. Many of them are very adamantly in favor of low carb and very low carb. I think maybe there is a link with aerobic fitness, endurance activities and greater tolerance of the "zone of misery".
Anyway, it seemed like that zone of misery didn't make sense. Why should it be such a sharp either/or dichotomy between the two states? But maybe it is related to our sedentary lives.
Female, 5'3", 50, Max squat: 202.5lbs. Max deadlift: 225 x 3.