B. hike to the top of the trail is what the writer is about to do. The writer glances up ( he takes a quick look upwards) at the summit ( the top of the mounatin) after evaluating some information. The lenghth of The Chilkoot ; the fact that the pass ( a road through the mountain) is some miles in elevation.
The underlined words indicate the direction the writer will take: to the top of the trail.