travel distances 你就理解为travel a long distances的省略版
楼上说有逗号是不对的,楼主的句子是正确的句子.
这是一个虚拟语气的句子,所以那里用 would have done.
that 后面的从句全部用来修饰distances.
翻译为：他们走了很长的路去工作,这段路长到就算是他们的父母来走也会气喘吁吁.
其实to work可以移动到句末变成They travel distances that would have made their parents gasp to work.