This out and back trail is only about .5 miles total and leaves from the main entrance into the Royal Palace compound from the Elephant Terrace. After passing through the main gate, run straight back along the path. Soon you'll see the temple through the trees. It is best to run this temple trail in the morning because the sun will be to your back and make for better photos of the temple.
When you reach the temple, the trail goes either way around. I went clockwise. You cannot explore the inside or top of this temple. Once around the temple, return the way you came in.