In my experience, it takes nearly a month for the serious "hiker hunger" to set in, as your body starts trying to eat itself.

I managed to eat enough on my AT hike to end at roughly the same weight as when I started, and was able to return to my career as a firefighter with very little loss of strength. The AT was my third thru-hike, and I learned some eating lessons on the first two. I carried over 4000 kcal/day, and pigged out in towns. I spent as much on town meals as I did on pack food for the whole hike. I try to tell myself I ate well but there was a lot of ice cream.

I have a theory that experienced hikers use less energy than average, maybe through better bio-mechanics, and maybe some weird genetic abilities. Some people walk and run with a smoother gait. Some people can tolerate fasting better than others.