He did not know that that ledge was only an hour or two's climb away and that before the light went they would probably have conquered two or three of these peaks.

【翻译】他不知道那座山的岩石架离我们攀爬只有一两个小时的路程远，而且在亮灯之前，他们可能已经爬越了两三个山峰。

away 是副词，只表示距离说话人“远”，作was 的表语。这跟下面的句子是一个道理：

The station is five miles away. 

区别是：一个是时间 X 爬山的速度；一个是具体的里程。

an hour or two's climb away：攀登一、两个小时的距离远。