_____ would have taken our ancestors days to reach,是定语从句,修饰先行词places.
但是,并非所有以place作为先行词的定语从句都在用where作为引导词.
从句的引导词先用关系代词还是关系副词,取决于关系词（即先行词那个词的意思）在从句中（注意是从句中）充当主、表、宾、定；还是充当状语.
如下面两句：
we talked about the place which we would reach soon.
we talked about the place where we would find some food.
一句中,定语从句which we would reach soon(在句意上完全等于we would reach the place soon),其中关系词用了which,是因为which（所指的即是the place）在从句中充当reach这个动词的宾语,故用which这个关系代词.
二句中,定语从句where we would find some food(在句意上完全等于we would find some food at the place),其中关系词用了where,是因为where (相当于“介词+which”,即相当于 at the place),在从句当中充地点状诘,故用where这个关系副词.
所以所提问题的句中,从句从句意角度,缺少 would have taken our ancestors days 这个谓语结构的主语,名词才能充当主语,地点状语不能作主语的,所以用关系代词which,而不能用作地点状语的where.
主语的意思是：我们一小时后能到达那里.从句的意思是：我们的祖先也许会花费数天才能到达那里.主从句合起来（强调定语的译法）：我们一小时后能到达我们的祖先许会花费数天才能到达的地方.
从句中的take,表示“花费”之意,用would have take表示“本会.而实际上没.”（所以句中翻译作也许）,to reach作目地状语,表意“为了到达”.