The descent into Nattai Gorge is called the Boxvale incline. It is a 300 metre, very, very steep, steel cable assisted descent to the bottom of the cliffs. I would recomment a decent level of fitness for the loop track, partly because of this steep and rough track and partly because the trail is quite over grown and hard to manage. I slowed to a crawl through most of the 2km that followed the Nattai river. Most of it was spent climbing over, around or under fallen trees or collapsing trail. Make sure you have a map with you also as it is not always obvious where the trail goes, especially when river crossings are involved.
Of course if you feel like challenging and/or punishing yourself you can also do it in reverse. This means after a nice 4 hour walk you can do the Incline as a steep climb up! This option took me about 5 hours to complete.