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.

Yup, I have never gone that far (but my summer hiking is entirely at high elevation with lots of climb) but I have never found anything like a failure point--I wear out because of time (not even daylight--I've made navigation errors that left me out there well past sunset), not muscle failure.