Best route would be to follow the cycleway which runs beside and under the M4 from Homebush (at the northern end of the Louis Savage Pathway) to Church St. It is not legal to ride on the M4 from Homebush to May's Hill (Church St on-ramp). West from Church St, the M4 shoulder is possible, and with peak hour congestion, maybe not too terrifying. It's pretty common to see cyclists commuting on the M4 between Wallgrove Rd and Parramatta (in both directions). If you're going to try a test-run on the weekend, you might find negotiating the off-ramps and on-ramps a bit more difficult than it will be during peak hours.
Just a warning word : over the last couple of weeks, the east-bound shoulder has been closed where it crosses Reservoir Rd - a detour through the servo's and back via the Reservoir Rd on-ramp has been necessary. Can't recall if it had re-opened yesterday or not.
If the M4 shoulder scares you too much, the next best option might be to take the Parramatta River Valley cycleway to Parramatta, then head thru Parramatta Park, around the southern side of Westmead Hospital to Old Windsor Rd, up that to the M7 cycleway, then back-track along that to Rooty Hill. Lots' more km's, but you did say you were doing it for training

Cheers