Thru-hiker.com has an entire section dedicated to nutrition for hiking and backpacking called "PACK LIGHT, EAT RIGHT©" http://thru-hiker.com/articles/pack_light_eat_right.php
To answer your question directly, for longer hikes 35% fat is recommended for optimum performance, while for weekend hikers a diet of 30% fat is sufficient. http://thru-hiker.com/articles/PackLightEatRight/fat.htm