Just show up and try to improve yourself. Be friendly.
Pretty rare I find one of these that are both around 3 miles (about my limit most days unless I'm at a park all day with breaks, my feet and sometimes lower back start getting pretty sore) and a more casual pace. I do attend those that I find, though. But that tends to be like 3 per year. I have to make up for it with solo hikes.
I don't think I've found any that are just 'go as far as you want and we'll meet you back at the local Starbucks' though, at least not near me.
Here's the next few upcoming, as an example:
"6 mile Hike Not a Beginner's hike" - (Tuesday midday)
"About 4.5- or 5-miles total" - (Tuesday midday)
"Walk about 2 miles" (but Tuesday midday and 45 minutes away, can't drive there, hike 2 miles, and drive back during a workday)
"We will be hiking 4.5 miles at a faster pace (3.75 mph) dependent on trail conditions." - (Evening, but fast pace and a bit too long)
"6.5 miles with shorter options" - (from the map it looks like the shorter option is "turn back around", it's one big loop. it's on a Sunday but it's also over an hour drive away from me)
No mention of meeting anywhere afterwards in any of those.
The other option would be to start your own.
My most local group was started as a Facebook group by a lady who wanted to run with other people. She lives on a country road and wanted to run with others in town, so she took over an existing but dead Facebook group. The time and schedule would be up to you.
This is pretty much her strategy, being intentional that you want, and welcome, people to join you.
I'm aware I could start my own. I once was the admin of a different type of Meetup group that had over 1000 members. It's just more way work than I have the time and energy for anymore, and also pretty expensive if done on Meetup.com nowadays ($200/year). For as infrequently as I'd host meetups it wouldn't be worth it.
I could also try to become a co-organizer of another group, but that would probably require me being able to go to enough of these for them to trust me to be a co-organizer.
It's certainly not impossible, but it's more hassle than I want to go through, for something that I can do on my own whenever I feel like it (and with my dogs).
Downtown Chicago is a bit far for me to drive for a run, but I'll keep an eye on it in case one of them ends up a bit closer to me. And maybe this will help someone else that does live downtown. Thanks for the suggestion!