I'm local to Cambridge / Kitchener. Haven't gone any further east on a bike than Dundas so I'm at a bit of a loss there.

I just think once you get to Kingston you start heading too far south by taking waterfront trail.

If you don't know these tips already - which you may:
1 - zoom out on Google maps with the bike map feature turned on (under advanced settings usually in top right)
take a loook at the general flow of the roads, see how they tend to slope at an ESE direction, leaving you with limited routes once you've gone too far south. problem would be finding enough paved routes (with low speed limits or less traffic) to make the trip easier on effort and still safe.

2 -search the bikemap.net site and see if it comes up with anything...