About 9 miles of moslty doubletrack with some short sections of old forest roads thrown in for good measure. According to Douglas County Parks and Recreation, there are currently 3 marked loops and several extra unmarked "side trails". The Blue loop is about 1.1 miles and easy. The yellow loop is 2.4 and a little harder but still relatively flat. The red loop is where the real fun is at 5.1 miles with some very punchy climbs that will certainly get your heart rate up. All trail loops are pretty well marked and even if you do shoot off on one of the "side trails" it will quickly bring you back to one of the three main loops. Pretty easy to navigate and reasonably well marked.
From Atlanta, take I-20W to exit 41, Lee Rd. Turn left and head south on Lee Rd for 3 miles. Take a left when Lee Rd dead ends into Fairburn Rd/Ga 92. After approximately 2.5 miles, turn right on Ga 92/Ga 154/Ga166. Continue another 2.5 miles and Boundary water Park will be on your left. Park in the far right corner of the parking lot in the rear of the complex. Trailhead signs here.
http://www.singletracks.com/item.php?c=1&i=4329