The only flat running in Durango is on the River trail, which is concrete. You can easily do ten miles on that. There are two excellent trail runs directly outside of town, a half mile jog from the center of town. They are both steep and very rocky, and go farther than you can run.
Actually, the best trail running in Colorado is in the Aspen area. There is much more variety and the scenery is a lot better than Durango. The cost of living for a tourist is about the same in Aspen/Snowmass as it is in Durango. Durango only has two worthwhile running trails; Aspen has at least 20 that are better than anything in Durango. Also, Telluride has many more trails and superior trails to Durango. I've lived in all three of these towns and run thousands of miles on the trails.