This from the Greenland Trail Web site.

The Greenland Trail 50K is a trail race located on the front range of Colorado . It was started with the intent to kick off the trail running season by offering a variety of distances to suit your running needs. Run in the Greenland Open Space of southern Douglas County , the Greenland Trail 50K strives to be the premier 50K in Colorado. The race is limited to 550 runners and has filled the last two years. So, please register early!

As of February 26, the race is 90% full.