Are you thinking locally or someplace you have to travel to?
I used to lead a ride around Lake Geneva a few times a year-- I've zipped the folder with the info and maps and am attaching it (see below). You can ignore the "ride leader instructions".
North Shore Century is next week which I think is from evanston to kenosha and back. I could use a riding partner for a longer ride early this week perhaps, I have tues/wedn off. For long rides, I usually work my way up from Logan Square to Foster and the LFP and ride south to 71st St and then back up again, or something of that variety.
A full loop on the LFP with going around Promontory Point and Northery Island will be around 40 miles. Depending on where you live, riding to/from the path could put you close to 50 miles. It might not be exciting, but it has lots of drinking fountains to refill your water bottles and bathrooms if really needed.
Another good option is to take Elston up to the North Branch Trail towards the Botanical Gardens. Depending on where you are starting from, you can make that a 50 miles trip. You can stop, relax, and eat at the gardens as well.
I rode 50.5 this afternoon/evening, to and back from the western suburbs. I took St. Charles between Maywood and Lombard and made growler-fill pit stops in each direction at Lunar Brewing in Villa Park (located on St. Charles). I tacked on an additional 5 miles on the Great Western Trail.
Going west through the city, I like Palmer/Bloomingdale or North Ave.