There really isn't anything to "treat". You just report the results for all the levels of race. If there is an interaction between race and distance, then there is an interaction - it makes no sense to say the interaction exists only at one level of race. That would be the equivalent of saying "Me are taller than women, but women aren't shorter than men". In addition, "significance" is a bad guideline for including or excluding a variable. But beware if any of your levels of race have small n. That could cause problems and you could deal with those by either combining categories or deleting some people (E.g. if you use the standard US Census levels of race, then, in most of the USA, there will be very small numbers of "Native Hawaiian or Pacific Islander" and "American Indian or Alaskan Native". In some areas there will be small numbers of other categories as well (not a lot of African Americans in Maine, for example).
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