I was born and raised in Chicago, and the North and South loop are very affluent areas. I would say the majority of the people you saw running down the Lakeshore trail - in that area- were residential runners, and some of those runners were probably from the Lincoln Park area running further South. There maybe some out of towners or visiting runners mixed in, but the running scene downtown is definitely dominated by the people who live in that area.
Also, I wouldn't necessarily call them hobby joggers, more like middle of the pack runners who like to keep in shape. Those individuals run just about every day, and they compete in a lot of endurance sports. They have the money to do so, and they probably get a lot of their gear from Fleet Feet Sports ( Roosevelt or Old Town) or Nike Town( North Michigan Ave).
There may be some competitive runners in that area, but I'm willing to guess that most of chicago's elite, competitive runners come from further north and further south.
Actually, it's quite stimulating to run on the lakefront trail , because the path is full of so many runners, and it is great for doing long runs and tempo runs. You don't have to worry about people staring at your, or yelling out to you, because everyone is focused on their run.
I will say this, and i don't know when it started, because it irritates the hell out of me, but a lot of runners in Chicago run on the sidewalk when they are not running on a trail or on the track. I grew up running on the roads(street), and running on the sidewalk makes absolutely no sense to me. The sidewalk is made of concrete, which is a terrible surface to run on, and it is filled with pedestrians. I think it started with running shoe stores and running groups doing fun runs and pub runs in the neighborhood, and it just escalated from there.
Next time you are in town, you should go running over in University Village, which is basically the campus for the University of Chicago. It's considered the West Loop area, and it is just a few blocks west of Michigan Ave. You will see more competitive runners in that area ( some runners from the high school and university track team) , and more of them run in the streets.