Dear Courier-Journal. 5k is a distance. If you're unfamiliar, it's 5 kilometers, which is roughly 3.1 miles.
Marathon also refers to a distance. At present, that distance is 26 miles, 385 yards or 42.195 kilometers.
In history, the distance has been as short as 24.85 miles. However, it has never been remotely close to 5k. The last finisher of the Anthem 5k finished 30 minutes sooner than the world record marathon time.
I do not expect the average person to understand the distinction; however, in order to report on a basketball game, you wouldn't allow the terms "extra innings" and "overtime" to be confused. "Home runs" vs. "Touchdowns?"