I enjoy running round airport environs to fill wasted hours inbetween flights. It is also a good way of getting in the right mood for the next flight

Directions: At YYZ take the shuttle to T3 and then (directions from inside the terminal) turn left and walk until you get to the end of the terminal. Don't keep on the approach road, but instead there is an exit off the sidewalk toward a small carpark. Walk diagonally across the carpark (follow your nose) and take the road that leads directly away from the airport and you can run alongside some wide relatively deserted roads for a few km.
Leave you bags in the left luggage.
Shower in the lounge (assuming you have access - if not PM me for a pass).

I vaguely recall there is a web site with "runs from airports" somewhere, but haven't done a recent search