Where is the orange entrance to Marathon in World of Warcraft?

After entering the Marathon sign in Desolace, you will find a cave inside. In this cave is the entrance to the Marathon instance.

1. First open the map in the game. There are two entrances to Marathon 1. Endpoint 2 in the picture below is one of the entrances (orange).

2. From end point 1 in the picture above to the end point in the picture below, you will see the second entrance of Marathon (purple).

3. No matter which entrance you enter the dungeon from, you can kill all bosses to complete the dungeon. The Poison Water Falls is the only way to enter the Tomb of Zaltar on the next level. After entering from the purple entrance, follow the red line in the picture below to reach the Poison Water Falls.

4. Finally come to the Tomb of Zalta and defeat all the bosses along the red line in the picture below to complete the copy of Marathon (the arrow position in the picture below is not well represented by the red line. In short, always stay at this position. Just go up).