I backpack often (usually 8-13% bodyweight in my pack) and during long summer days I can comfortably push well into the 30 mile per day range if there isn't too much vert to slow my pace down. My feet get sore, brain gets tired, and I run out of daylight well before any sort of muscle failure in my legs. If you aren't used to walking from sunrise to sunset doing so would build muscle, but your time would be better spent on a progressive overload leg routine in a gym.

I used to persistent hunt to failure, ended up with bulky calves and tibialis.

Check anybody that has done the AT.