It's possible to ride from Kingston to Toronto in two days via the Waterfront Trail.
It's possible, but it means two extremely long days. The distance is about 280 km.
You might also consider Loyalist Parkway in Prince Edward County.
The Loyalist Parkway is great for cycling. There are wide cycle lanes almost everywhere. But the Parkway gets a lot of traffic. The charm of the place for cyclists is the hundreds of side-roads and country lanes that crisscross the County. For example, you can cover huge distances east-west along Gilead, Danforth, and Swamp College. You will see more cows than cars.