If you're intending on hiking the Overland Track, you need to pre-book it. Throughout the peak season between October 1 and May 31 it typically books out months in advance and needs an authorization to be purchased.
During the off-season you still require to reserve your area, however the license is complimentary.
When you book your Overland Track, you schedule your start date, which's it. You can remain on the track as long as you desire, and there's no time limitation to exit the trail. You just can't go backwards at any point.