+10 on avoiding King Georges Rd. Just way too many bad-tempered drivers in that part of the world, and your slow speed on those hills at South Hurstville will definitely light their fuses.
Might be better to find a way (e.g. via Hurstville Rd/Boundary Rd) to Mortdale then through to Riverwood, pick up the M5 off Belmore Rd, take it to River Rd Revesby, then start heading north: cross Milperra Rd at Mons Rd (on foot, or by taking footpath on northern side from River Rd), then left at Eldridge, Right at Olive, left at Augusta, right at William St, follow all the way to Hume Hwy at Yagoona, continue onto Auburn Rd, all the way to Regents Park, left across railway line, right at Park Rd, left at Rose Cres which turns right to become Chisolm Rd. From here you are on the Ducks River cycleway. Follow Chisolm all the way to Mona St, left onto Mona, then turn right onto the cycleway just west of the bridge over Duck River. Take cycleway to Seventh St, then right onto Factory St.
Assuming you are confident in traffic, left at William St, right onto Memorial Drive, which becomes Railway Parade, turn right at Bold St. Left at Parramatta Rd, right at Albert (using northern footpath from Bold st), left at Prince St, right at the end (Harris), down across the stormwater canal, up to M4 cycleway, and keep going north up to Harris St, left at Macquarie St. This will bring you up to Parramatta CBD and Parramatta Park. Will leave it to you to work out the rest of the way to Toongabbie from there.
Map hereedited: 14/06/11 7:35 pm, link to googlemap included.