Second go at posting on this thread. Last night, there were some other posts on this thread - one from commurider, one from someone else and one from me.
The M4 option now posted by fringe_dweller is safe enough that I see plenty of other cyclists commuting on it in peak hours - in both directions. Be cautious at the off-ramps and on-ramps, but they shouldn't be terrifying because the peak-hour crawl will slow them.
Just a warning: over the last few weeks, the M4 shoulder has been closed at Reservoir Rd for bridge repairs. A detour thru the servo's was necessary. Can't recall if it was open on Friday evening.
If you're really worried about it, you could go a longer route via the Parramatta River Cycleway from Homebush to Parramatta, then thru Parramatta Park, around the southern side of Westmead Hospital, up Old Windsor Rd (shared path/cycleway) to M7 cycleway, then back along that to Rooty Hill. Extra km's, but you did say your goal was to do some extra training

Cheers