Beginning at the Forest Gate, it ran some 180 miles from there to the end of Mirkwood near Thranduil's caves. The Enchanted river ran through the path nearly three quarters of the way through. It was somewhat narrow but fairly level with a few up and down here and there.
The path was wrought by the Elves in the Third Age of the Wood-land realm probably when Sauron occupied Dol Guldur and the forest became filled with evil things. Being the safest route through northern Mirkwood, Gandalf chose this way for Bilbo and Dwarves to take to the other side during their quest to Lonely Mountain. It may have survived into the Fourth Age.