I did something similar with a DL ticket connecting to an AA ticket. The DL flight from DTW was delayed by >3 hours, which made my connection very close, so I did the whole process as quickly as possible. Transiting YYZ is a pain, to say the least. Indeed, you have to exit security and essentially start your new ticket from the outside. I have Global Entry, which expedited things, but it still took about 45 minutes given all of the walking and security/immigration/customs steps. Hopefully your arriving flight is on time!