I would just use kilometres, it's a fairly world-wide unit of measurement, known even in the English speaking world. Most people have a vague idea of how far it is even if they can't easily convert from kilometres to miles in their head - I imagine even more so if they are interested in military fiction.
Besides, don't (Didn't) even the American military use the slang "klicks" to mean kilometres? You'd often see it used in American media by American characters for an American audience - and they don't seem too bothered about the audience knowing exactly how many miles that is (not to mention what the hell a "klick" is)
Not sure if they did as far back as WW II but your audience are modern English speakers.
If exact distance is a factor, you could always add in extra information like ("so it will take fifteen minutes to get there").