Hi there, this is a slightly strange phrase:
English often uses the structure ‘X by X’ with small units of time or distance; e.g. inch by inch, minute by minute, and similar phrases such as bit by bit or little by little. In all these the meaning is that something is being achieved slowly or with difficulty.
I guess that mile by mile is being used in the same way, but as it is a large measure (unless the context is something like space travel?) it sounds slightly strange!
Hope that helps, Helen