I grew up in Geelong, so the GO Road area is quite familiar to me.
For some shorter walks, I'd suggest places like Blanket Leaf Gully, Melba Gully (although better to go at night, to see the glow worms), Lake Elizabeth (near Forrest), Stevenson's Falls or Little Aire Falls. You can also link some of these up for longer walks. Try the Vic parks and Wildlife website for maps? There used to be quite a good map of the Otway area, but I can't recall what it was called. Try some of the outdoor shops in Geelong for maps/advice, etc.
Near the 12 Apostles (which is little more than a crowded public dunny these days

) is Gibson's Steps, where you can walk down the cliff face and onto the beach. Also, just opposite the turnoff to Glenaire Farm, if I recall correctly, is a gate (on the ocean side) that you can go through, and drive up the hill just past an old quarry. If you park there and walk up, there's a path through the dunes down to a beach that's very secluded.
If you turn off at Mogg's Creek and head up towards the old Girl Guides camp (Tallawalla), there used to be a track there up the Gentle Annie hill, which has pretty speccy views.
One of my favourite camping spots is on the Otway peninsula, at Blanket Bay. Turn left off the Ocean Road at Otway lighthouse road, after 9ks, take a left at the Blanket Bay road. Sheltered campsites right on the beach, great little cove for swimming, protected from the swell by a reef.
frank_in_oz is also very familiar with this area, I do believe ..?