Wherever you go in the Black Hills, the Mickelson Trail is never far off. This 108 mile rail trail extends throughout the west end of South Dakota, from Deadwood to Edgemont. It’s the perfect choice for a long run, and a great way to explore the scenery of the Black Hills. Runners can hop on at any point, but some of the highlights include the historic town of Deadwood, the city of Custer, and the various creeks, tunnels, and bridges along the railway. The path is mostly gravel, but there are some paved sections through places like Custer and Deadwood.
The accompanying trail map has excellent details on trail segments and distances.