Take the Go Train to Rouge Hill (Scarborough), hop on to the Waterfront Trail, and head east. Most of the route is on quiet backroads and trails. (Maybe 5% of the time is on South Service Road or Highway 2; but even then there is usually a wide shoulder.)

If you really ride as fast as you say, you will arrive in Prince Edward County at the end of Day 2. Visit wineries, go swimming at Sandbanks Provincial Park, explore backroads, etc. At the end of Day 3, ride to Belleville and take the bus back to Toronto.

(I do not ride as fast as you, and I reached the heart of the County at the end of Day 3.)

There are grocery stores, restaurants, motels, hotels, and bed-and-breakfasts all along the way, although many of these amenities are north of Highway 401 between Toronto and Port Hope, which means taking detours off the Trail. The Waterfront Trail mostly hugs the shore of Lake Ontario, and it is an exquisite route!

Also note that you may have trouble finding overnight accommodations between Coburg and Brighton.