These Inca roads were made up of two north-south highways and many small roads crossing the mountains east to west.

east 是名词，意思是“东、东方”，在这里用来表示方向。它常与 west（西）搭配使用，构成 east to west 这一表达，意为“从东到西”，用来描述道路或物体的走向。