Seems like everyone in you meet lives in either the Plateau or Mile End. If not, somewhere bordering there. There's this stigma surrounding living right near Concordia, as if you'd be trapped in a constant frat party. I assure you that's not true, and in fact it's a pretty great area to live in!
Centreville is a "student" area, meaning restaurants know that's where to cater to people who don't want to dish out a lot of cash. You'll be getting the same deals set out to lure in students, but all the time. There's $2 falafel, cheap takeout all over, and student coffee deals in pretty much every school building.
If you're a student, you get a pretty sweet discount at Concordia's Le Gym. It's done by semester, so you can either sign up for a 4-month membership or enrol in a fitness class! It's way easier to get yourself to the gym if it's within walking distance!
Getting to campus from the Mile End can be super annoying. If you're already downtown, it'll be a lot easier to get yourself to your morning lecture, and you'll have less of an excuse not to go do your readings at the library.
You've been on Saint Catherine. There's literally every store you could ever dream of, all in a row. You can pop into H&M, Forever 21, Club Monaco, Zara, ADIDAS, a bunch of great shoe stores, cafes, and more! You'll never miss a deal!