When I first moved to Chicago and started biking again regularly, my go-to long ride was an up-and-back on the Lakefront Path. That is still a great ride (about 35-40 miles door to door, round trip), but the northern section of the Lakefront Path is overly crowded during the summer, even at early morning hours.
These days, when I don't want to think about where I'm going to ride, my go-to Saturday ride is Chicago's North Branch Trail. A little less than 20 miles of paved trail through the wooded Cook County Forest Preserve, the trail loosely follows the North Branch of the Chicago River, winding its way north from N. Milwaukee and W. Devon Avenues, along the river and then meandering through the Skokie Lagoons until it reaches the Chicago Botanical Gardens.
In the Spring and Summer, the Trail is shaded and grants some relief from the heat, although the humidity can be just as bad or worse because of the river and lagoons. It is a relatively wide, flat trial, with minimal street crossings and two overpass bridges (no dismount needed). It is not a trail on which you can (or at least should) get up to racing speed, but is relatively fast given the well-paved surface and relatively low traffic level on weekend mornings. You share the trail with other cyclists (mostly recreational bikers on hybrids, with the hard-core trainers sticking to the faster and less crowded streets), walkers and joggers (there seems to be an group stroll on Saturday mornings at about mid-trail for "older" Asian ladies and gentleman) and the occasional stroller and dog. There is also a gravel horse path that parallels the trail, but I have seen very few horses in the numerous times I have ridden the trail.
Once you get to the top of the trail, you can access the Chicago Botanical Gardens by bicycle, but just the back/service roads, with almost all of the actual gardens being off limits for bike riders. For a shorter, up-and-back morning ride, the Botanical Garden makes a good turnaround stop, with a nice bathroom/rest area with some vending machines, bike racks (although the racks are not well designed to work with U-Locks) and shaded benches on which to sit a spell and savor that Powerbar or energy gel at your leisure. For longer rides, the Botanical Gardens makes a good jumping off point as well, with the Skokie Valley bike path and the Green Bay Trail/Robert McClory Bike Path only a few blocks to either side of the Garden's North entrance.
Up and back will give me 44 to 46 miles, depending on the exact path through the Skokie Lagoons taken. This always seems a little short to me. If I have time, I will jump up to the Skokie Valley Bike Path and then come back via the Green Bay Trail to the North Shore Channel Trail, which includes a sculpture garden. This will bring in the ride at somewhere in the 65-70 mile range, and to my mind is just about the perfect length for a weekend ride. But for those mornings when I did not get out the door early enough, an up and back to the Botanical Gardens is a good fit.
The only drawbacks to the North Branch Trail are typical for in-city trails in general. The quickest route to the Milwaukee/Devon trailhead where I generally start is by Elston Avenue, which merges into Milwaukee a few blocks before the parking area for the trail on Devon Street. Elston has a good, wide bike lane and is a fast street, especially if you can catch the right rhythm for the stop lights, but it is not what I would call aesthetically pleasing for the most part and can be traffic-heavy at times. The other drawback is as I mentioned above - the North Branch is really a recreational trail, not a route for intensive training (if you are in to that sort of thing). It's a little too crowded, too twisty (although that can be fun) and too narrow for riding much above 18mph, except in short bursts. But since I rarely ride faster than that other than for short bursts, this is fine by me.
The best part of the North Branch Trail is how isolated parts of it can seem from the city. You can really feel like you are riding through the wilderness in sections - especially if you are lucky enough to catch a family of deer in the early morning mist or get a glance of a river turtle sunning itself on a muddy bank.
Sunday Ride Stats (August 29, 2010):
North Branch Trail via Elston/Milwaukee (up-and-back)
Bike / Load: Trek 2.3 / high tail w/camera
Total Miles: 44.5
Ride Time: 2 hours, 42 minutes
Average Speed: 16.5 mph