The most important thing in a relationship if you want it to go the distance is being kind.

一段关系中，如果你想让它走得更远，
最重要的事情就是要善良。

1. The most important thing in a relationship is ...
在一段恋情或关系中，最重要的是。。。

The most important thing in a relationship is trust.
一段恋情中最重要的事情是信任对方。

2. go the (full) distance 坚持到底 == last the distance
Don't volunteer for the job if you can't go the distance.
如果你不能胜任这份工作，就不要自告奋勇。

Do you think Greg will go the distance this time?
你觉得格雷格这次会坚持到底吗？

It takes time, so you have to be patient, you have to be willing to go the distance.
这需要时间，所以你必须有耐心，你必须愿意坚持到底。

You can't go the distance if you are not on the road.
如果你不上路，你就不可能坚持到底。

Only those who can take the heat will go the distance.
只有能承受高温的动物才能走得更远。