Miles or Kms?

My friend I run with talks everything running in miles because she's from the US...naturally she says it is miles...well sometime I forget and she tricks me in to running a lot longer then I think because of it. Silly me! Today was a nice 8 miles. I thought it was kms...until we started out on the route. That's ok but in the winter she said we were doing a 18 miler and I thought kms and boy was I off...It's funny how I like to mentally prepare myself for every run and how off I get when the distance changes. I need to work on that. I think it's because I pace myself in my own mind...