Both the kanji in this word use the kun'yomi, which you learned when you learned the vocabulary. When it uses this reading, it sounds casual, and is common in conversation. It can also be read using the on'yomi, though, in which case it has a more formal feel to it. That's typical of on'yomi, which tends to add formality to a word!
I walked the mountain trail with my puppy.
From there on, it becomes a mountain path.
Beyond that point, the road enters the mountains.