Start up the dirt road about a mile or two and you will see the trailhead for Elkhorn loop a little bit after the climb levels off and then starts downhill. Then follow the markers that say Elkhorn loop. It will kick you back out onto eht street where you started your climb. It's a fun run but not very technical. Really good views.
Take 21 to Idaho City from Boise Go past Idaho city about 15 miles. About half way between Idaho City and Loman there's at switchback with a large parking area on the left and a trail head with a map of the area on the other side of the road. This is a whole network of trails and this is where the Elkhorn loop is.
http://www.singletracks.com/item.php?c=1&i=2224