This is as far as we go.

as far as 不是真正的连词，而是比较结构。第二个 as 是比较连词，第一个 as 是副词。理解句子要从源头解决。比如说：我们走路前行，到了某地需要停下；或者公交车到了目的地。就可以说这句话，意思是：我们就到这里了。

this 指 this place。句子结构演变如下：

This is far.  这里已经很远了。

This is as far as we go.  这里跟我们要去的地方一样远了。引申：我们就到这里了。

有的语法书，笼统地把 as far as 看作固定搭配，其本质还是比较结构演变来的。例如：

（1）as far as 视为一个短语介词，后接地点名词。表示“远至......地方”。

We walked as far as the railroad station. 我们走到火车站那样远的地方。

My garden extends as far as the river. 我的花园一直伸展到河边。

The ladder won't quite reach as far as the window. 这梯子很难够到窗子。

We drive as far as the utmost end of this city. 我们开车到此城的边界。

I've read as far as the third chapter. 我已读到第三章了。

（2）as far as 用作连词，接句子。意为“跟......一样远”，还表示情况达到某种程度，意为“远到...；就…而言；在…范围内”。

The desert continued as far as the eye could see. 沙漠一直伸展到视线的尽头。

They went as far as they could. 他们尽了最大努力走了那么远的路程。

The ocean stretched as far as they could see on all sides. 海洋十分辽阔，他们极目远眺无边无涯。

还有，as far as I know, as far as I am concerned, as far as I can see 等，也是跟主句比较产生的固定结构。