An easy way to do it and get a rough idea is to multipyl the kilometres by 6 and knock off the last digit; eg. 100k times 6 = 600, knock off the last digit (0) and you get 60 miles.
After reading sandy's response, I realize that I should have said multiply by 6 and move the decimal point one place to the left. Eg: 16k * 6 = 96, and moving the decimal point makes it 9.6 miles.