The "K" in "100K" can stand for a few things; it could mean kilometers, indicating a 100 kilometer foot race, commonly called an ultra-marathon. Alternately, it could simply refer to the prefix "kilo," meaning one thousand. This means that "100K" stands for 100,000.
When K refers generally to the prefix "kilo," it is in instances where a large number is being used. For instance, a salary of $100,000 per year could be referred to in abbreviated fashion as "$100K."
The 100-kilometer ultra-marathon, equivalent to approximately 62.14 miles, is one of the more common lengths of that type of race. Other common lengths include: 50 kilometers, 50 miles and 100 miles, although the 100-kilometer race is recognized as an official world record event.