What is a 50K?? Fifty kilometers is roughly 31 miles. The StumpJump running race took place in Chattanooga, TN, and was almost entirely on trails. Running distances beyond a marathon (26.2 miles) are called “Ultra-distance“. The most common distances in this category are 50k, 50 miles, 100K, 100 miles. (I guess race directors are either on one side of the …