This is a great trail to run early in the morning. The trail does not enter the temple but you'll pass several different gates and breaks in the outer temple wall that will give access. I recommend completing the run then go in the temple to explore.
This trail starts from the west gate of Preah Khan. Follow the wide path to the main west entrance. Before the short stone causeway, you'll see the trail going in either direction. It is best to turn left doing the run in a clockwise direction. The trail follows an outer wall and has several gates and openings to explore the temple complex. However, it is best to explore inside before or after this loop run. The trail rounds the northwest corner in a very shaded area. The stone ruins have a deep green moss growing on them that is great for photos.
Continue west to the north gate. Round the north gate to a nice section of trail as you round the northeast corner. This east side has less forest and on the southeast corner is a large tree that has grown on top of the portico and large roots make their way around the windows to the ground. An awesome sight.
Around the southeast corner, the trail is well-defined but gives the sense it is more remote when it is not. Pass the south gate and finish the loop after rounding the southwest corner. Follow the path out the way you came in or explore the interior of the temple complex if you did not do that first. There is plenty to see!