I had to be selective about which transit routes made the list. There are night buses that cover a greater distance than the city's longest line, the 54A Lawrence East (the 300A from Pearson to Warden station along the Bloor-Danforth route is a 75.97 km round trip, for example,) and some weekend routes that disappear on lengthy tangents, depending on the day of the week or the time of year. Both were left out.
The distances noted below represent the length of a return journey, because that's how the TTC measures the distances of its routes in its annual service summary (some routes take a different path depending on the direction.)
Here are the 5 longest transit routes in Toronto.