UP Express will drop you off at Union Station which is where the Canadian is departing from. I took UP Express couple of weeks ago from YYZ into Union Station and it was a breeze. Just follow the signs for "Union Station" once you get off and it will take you into the main station building which is where Via Rail departs from.
Also, it took about 30 minutes for us to clear immigration and customs which includes the time to collect our checked bags.