Fat is the ideal energy source for life's daily activities; whether walking, working, or running a 100-mile trail race, our bodies prefer to utilize fat as an energy source. Carbohydrates only come into play when you're doing repeated bouts of high intensity exercise like sprints or intervals. In those situations, it is helpful to supplement with a high carbohydrate drink or bar.
If interested in learning more about fat oxidation and low carbohydrate exercise we suggest reading the following book:
Volek, J. Phinney, S. (2012) The Art and Science of Low Carbohydrate Performance. Lexington, KY: Beyond Obesity LLC