It's ok to use "last mile may be hardest" as a title of a news report.We may not be able to catch the real meaning of the report at the very first glance of it, however, we can read the report so that we can figure it out. If it's used as a translation for a Chinese idiom , it's another story. Without explanation, how can you make yourself understood by the listeners? If you explain it, then it will take a bit too long for us to do the interpretation, isn't it?