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

It's common to use the expression * by * to refer to a process measured by the word inserted in this phrase. Example: We are getting closer to Los Angeles mile by mile . (Each mile we drive brings us closer to our destination.) Inch by inch, he crawled out of the plane wreckage. Day by day, I am learning more and more English. I am becoming fluent little by little.

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