So what about the South Coast track?
Start at Cockle Creek and go in for a couple of days - as far as you want to / as time permits, then back the way you came.
This gives you an insight to what you're in for on the return journey, thus you can judge how far to go. Might I suggest Osmiridium Beach or somewhere like that?
There's a good map you can get on the South Coast walk, it shows where all the campsites are and has some great info on the back of it. I'd recommend getting it laminated and roll it up, seems to help my maps last a long time. Typical maps a bit under $10, and about the same to get them laminated.
South Cape Rivulet to Granite Beach is a tough full day so be ready for that. I'm sure that would have been mentioned somewhere here if you do a search.
This is a well defined track, yet a very interesting and challenging walk. It involves river crossings, MUD, hilly terrain, beach walking, forests, spectacular views - a bit of everything.
And if the weather turned, no need to go any further as there is so much to see in this part of the world - just to be there and appreciate the wildness.
A quick edit - mud is mainly on the section South Cape Rivulet to Granite Beach though some boarding has been done.
Also be mindful of the South Cape Rivulet crossing - if it's fine when you cross then it buckets down you may well be trapped on the wrong side and have no option but to wait for it to subside.
That can take some time.
When I crossed in November '08 it was waist deep and fairly quick. We would not have wanted to cross the evening before with the standing waves and the very fast current -

- South Cape Rivulet with standing waves