Hi Genesis, the Deep Creek Circuit Hike is quite good, but only half a day maybe (12km). I wrote some blog entries about these walks a couple of years ago - if you want some GPS files I can send them to you -
http://jez-hiking.blogspot.com/2007/12/its-worth-that-5-minutes-of-planning.html. There's not a lot of info there, but some photos and maps to show you what to expect. I got a flat tyre on an isolated road and had to walk 10km? 15km? to get help (I had no spanner, I learnt a good lesson). Fun times.
The walk along the Heysen Trail from Cape Jervis to Victor Harbor is quite good and takes most people 4 to 5 days, the section particularly through Deep Creek is steep and many find a reasonably day's walk to be 15km, perhaps 20km is quite far. Depends on how you like to walk. A shorter option might be from Cobbler Hill to Tapanappa or Balquhidder. If you start at Cobbler Hill it might be worth walking down to Blowhole Beach.
The link to the Heysen website
google map will show you the huts and campsites (facilities, coords, photos):
Cobbler Hill, shared with car campers
Eagle Waterhole, open hut with bunks, walk-in only
Trig, shared with car campers, but an open hut could be used in inclement weather or quiet times, the campsite is located about 300m south of the trail, the hut is located at the southern end of the camping areas
Tappanappa, shared with car campers
Tunkalilla Beach, no facilities - no toilet, no water tank - the campsite has recently been moved to midway along the beach, but the campsite is not marked in anyway (will be marked sometime soon)
Balquhidder West Camp Site, no facilities - no toilet, no water tank, but adjacent creek, sloping ground though - this campsite has been closed but you won't see any indication of this, there is still a camping sign (the govt dept is a bit tardy), a new campsite called Balquhidder Camp Site further east has yet to be opened on newly aquired govt land
Waitpinga, shared with car campers, toilets, hut (officially no camping in but if it is quiet and the rangers might not mind hikers anyway)
I've also camped at a beach called Sheepies Beach or Sheepies Gully which is quite nice, between Balquhidder and Parsons Beach. No road access. It is at longitude 138.450322 and latitude -35.629575. There is a KML file attached. Some other beaches further west are stone beaches.
I've walked this section a couple of times. I consider myself a fast walker, last time I did it though I did it with a couple of medium speed walkers, there is a blog entry here with some walking time statistics from a GPS unit:
http://jez-hiking.blogspot.com/2008/03/at-least-i-would-die-happy.html (the reference to Unnamed Beach or Coolawang Beach refer to what I have later been informed by surfies to be Sheepies). Doing the whole distance from Cape Jervis to Victor in four days would be feasible for me.
You will also find GPX files for your GPS of the Heysen Trail at
http://www.heysentrail.asn.au/heysen_trail/maps.php.
Another transport option is a bus service operated by Sealink Ferries between Victor Harbor and Cape Jervis 5 days a week -
http://www.heysentrail.asn.au/heysen_trail/transport.php,
http://www.sealink.com.au/