There is a bike path along side the southern expressway from Darlington to Old Noarlunga (passes near Christies Beach). After that you will have to take the Victor Harbour road, past Mclaren Vale. It is a good open/wide road, so there is enough for you a other vehicles. Its probably about 50km from Christies to Victor, but very little flat areas, all moderate hills.
If you are staying in Victor, a great ride is from Victor to Goowla and back. I did it on Melb cup weekend, about a 60km round trip, lovely ride with great scenary.
If you need any more help let me know..