Math. That’s why. 8 kilometers is 4.97 miles. Add in the margin of error for road races, and you’ve got yourself a nice, clean 5-miler; but 8k sounds cooler. And to those who don’t run, saying you ran a 40:08 8k sounds faster than 40:08 for five miles. 8ks are pretty rare; 5-milers are too. The Double Five & Double Five Challenge (scheduled for April 9, 2016 in Lake Oswego – registration opens October 1)…