Any basic wired cycle computer will have a total miles function, @DragonsLair; they're pretty inexpensive like the one Al suggests. It won't tell you what you've already done in miles but will give you that info going forward plus have a trip odometer for each ride that you can clear for the next. The wired options bypass problems that can happen with energy field interference coming from the motor.
The Sigma mentioned above will show your total miles as well as the trip distance, trip time, total time, clock and speed. It is a wired model (recommended) , detaches instantly and uses a button battery. It is small but easily readable and simple to set up.
There are other brands, but I liked this one so much I bought another one for an old non-ebike.