Long before sth. 是在sth.(一件过去或现在发生的事)的很久以前。
For example: Long before the ancestors of human beings began to exist, the Earth was dominated by dinosaurs. 在人类的祖先开始存在于世(这个时间点)的很久以前,地球的统治者是恐龙。
Before long 是“不久之后”的意思。
For example: I hope to take you to the altar ere(before) long. (From Pride and Prejudice, Mr. Collins said to Elizabeth) 我希望不久之后就能带你走向神坛(结婚)。