The phrase way back has two primary meanings, as others have pointed out.
Using way back can be used to refer to a long time ago, such as "way back when." In this instance way is describing the word back as being very far back.
How far back was it? It was way back.
This meaning could also be applied to distance, meaning that something was a very long distance back on a path. Using way in this sense is fairly colloquial, not often found in formal writing.
The other use of way back is to refer to a specific path taken. In this case back is modifying the noun way, and often includes a destination.
Which way did he take? He took the way back (to the house).
This implies that the path in question has already been traveled once, but you are now returning.
Your case more closely relates to the second definition, though most people would not use it that way. Instead, I would simply say
Have a safe trip home!