The Lantern Festival was on what the Korean's call a "Stream".  The "Stream" possibly better called a "river" was covered in the 1960 by streets.  It was "uncovered" sometime in the early 21st Century and now as you walk along it, there is a feeling of a city park.  The guide said the temperatures in Seoul dropped by 2 degrees Celsius once the water was exposed.  I cannot tell you the length but it probably would have taken about 15 minutes to walk (so maybe just under a mile) if there hadn't been crowds of people