Looking at the maps I realized I misread the sentence.
Cascade Locks is on the Columbia river.
The entire passage means.
From where she was she had 142 miles to get to her "final box"*. Once she reached that
point she only had 106 miles left to hike: Olallie Lake to Cascade locks - 106 miles.
The Timberline Lodge is the midpoint between Olallie Lake and Cascade Locks -- 53 miles.
* You've probably figured out what she means by "box" by now, but just in case..... hikers mail provisions to themselves at stops along the trail so they don't have to carry too much. They pick them up at post offices. The provisions are commonly known as a "box".