5K races allow Church volunteers to distribute t-shirts with a logo that was designed by some kid in a high school graphic design class. This shirt will definitely contain a reference to Hebrews 12: "Let us run with perseverance the race marked out for us." Of course, by "race," they're referring to a distance of 5K, and by "marked out for us," they are referring to street course that has been marked out by church volunteers and local police officers (or at least the cops who are also deacons at the church).
The measurement of the race in kilometers as opposed to miles allows the white Christian to feel more internationally plugged in. Where possible, white Christians always prefer measurements in the metric system (SWCL fun fact: the average white Christian communion uses precisely 6 milliliters of grape juice!). Besides, "5K" sounds a lot better than "3.10685596 Miles ." For those in the congregation that are out of shape, the 5K ignorance is bliss. An unknown distance allows for the faithful fatties to attempt to run the race (albeit with limited perseverance).
Furthermore, there's always the remote possibility that a member of the congregation will interpret "5K" to mean 5 thousand- as in a $5,000 donation to the church. It's not very likely, but the church can hope. It is for this reason that some ambitious white Christian churches attempt a 10K race.