Go back a long way
Definition 1 of 2
Pro Tip 1/3Personal Connection
Indicates a close and possibly nostalgic relationship between people. 
Seeing you again after all these years reminds me that we go back a long way, my old friend.
Pro Tip 2/3Not for Strangers
Use it when referring to a relationship where individuals have shared past experiences. 
My mentor and I go back a long way; he's known me since my first day on the job.
Pro Tip 3/3Shared Experiences
Implies a history of common experiences or events that connect people. 
We go back a long way, to the times we fought side by side on the field.