I have no idea why they used the "Great Western Trail" for both sections, seeing as how they're not connected, like at all. All of the "old" paths/trails follow what used to be railroad lines, as the ones that branch out of Wheaton, which you rode today. Even the GWT from STC to Sycamore follows an old rail line, and when you get out to a few miles from Sycamore you can see the old mile marker signs. I have photos of them, somewhere.
The ride you did today is the "big" triangle, which I did back in 2014 before I had any kind of tracker. The "little" triangle is taking the GWT from West Chicago into Elmhurst where it ties into the Prairie Path, the ride the Prairie Path back out to Wheaton, and then take the Elgin spur back up to West Chicago. I've done that a few times, but the further you go east the more road crossing you have (like every block), and the more trail traffic there is. It gets really annoying, really fast.
I've taken the Prairie Path east all the way to the terminus, at the Forest Park train station.
If you want to go for distance, take a look at the I&M Canal Trail going southwest. I usually start at the Hollywood Rd park, instead of where it actually starts (that branch) in Rockdale. Much bigger parking area, seems more safe. You can ride that all the way to La Salle-Peru, about 110 miles each way. The section from Morris to Marseilles quick degraded though. It started nice at Gebhard Park, but quickly turned into a single file path, about 12" wide, with weeds hitting my knees. The area was cleared for a good 14-18', but the trail was not maintained. At some points there was pretty much no trail, you were just riding on grass. West of Seneca was terrible, large and loose rock gravel. Eventually I abandoned my ride at Marseilles, crossed the river, got some ice cream in Illinis State Park, and rode the country roads back to Morris. Added a good 10 miles, but it was better than the shitty trail.
No idea if it has been improved since then. That was summer of 2021, IIRC.