With all due respect, fantasy shouldn't be about following standards. Fantasy should be about creating your own.
I never said it was, but not everything needs to be completely unique. Miles might sound a bit modern for a fantasy series but leagues is an easy to grasp measure of distance which has that medieval feel. Personally I'd advise against making up your own measurements unless the culture that uses them is truly very strange or unusual. I'd rather a fantasy author measured distances in miles or leagues than say Zanthums or some other made up unit. I can grasp what the former two actually mean so when I'm told there's a thousand leagues of desert between point A and B, I can think wow, that's a lot of desert these guys have got to cross. Tell me there's a thousand Zanthums of desert and I won't have a clue. When it comes to little things like this I'm more likely to sigh at the author for trying to be a special snowflake rather than praising their originality. There are more important things to be doing imo.
Just because they are speaking English in the book, doesn't mean they are in the actual universe itself. Think about it, anybody can write in English and say they are speaking another language but is translated into English for the reader to comprehend. Like a English for the reader but something else in the fictional reality.
I know that, that's what I'm saying. There is no English in Middle Earth, but the reader/viewer sees/hears English anyway simply because there is no way for the author to translate the entire book into whatever language the author decides everybody in the Shire speaks. No one would be able to read it because it's too confusing. Certain concepts such as day and night, feet and arms, etc. are universal. How many fantasy universes have horses, a real life animal? So real world measurements can also be part of a fictional fantasy world. I've read plenty of fantasies where they talk about "miles", "kilometers", "hours", etc. It's less confusing that way.