After the 507, take Glamorgan Rd which is just northwest of it. Once you get to the 118, then whichever way you turn, you will find some fun.
If you go right at the 118, you can hit up Essonville Line, Loop Rd, Elephant Lake Rd, South Baptiste Lake Rd, Lower Faraday and 620/504. You can go further east towards Calabogie as well, but that will take you a little over a 600km loop.
If you go left at the 118, very soon you will be in the Haliburton area which has Harburn Rd, Haliburton Lake Rd, Kennisis Lake Rd, Kushog Lake Rd, Hwy 35 and much more.
The 28 and 118 both have excellent pavement and are nice roads to ride, but are generally fast sweepers than anything remotely technical.
Doing a 600km loop from Oshawa and you can hit up some really nice roads.