▲是的，句子中的介词用得不对，应将 to 改为 in，即说 go hiking in the mountains。句意：

If the weather is fine, we’ll go hiking in the mountains this weekend. 如果天气好，我们这个周末就去山里远足。

▲类似地，下面句子中的介词也不用 to，而是根据情况用了 in, on：

I went swimming in the river. 我去了河里游泳。

They can go fishing in the lake. 他们可以去湖里钓鱼。

It was so cold that we were able to go skating on the lake. 天气极冷，我们能到湖上去滑冰了。