As part of Mainly Marathons, the Southwest Series Day 2 Half Marathon is one of 75 races across the United States, this one taking place in Durango, Colorado. Road Runners. With a smooth paved path with short bouts of gravel trails, this half-marathon course is a looping course with a total of five (5) laps. Taking runners through the Riverwalk trail, you can expect tree-shaded paths, views of the river, and cool temperatures of early winter. Starting at the Fairgrounds in Riverwalk Trail on 2500 Main Avenue, this simple course design takes runners along Riverwalk Trail for a total of five up-and-back loops.