My friend and I started out from the Western suburbs (Wheaton area), so I'll comment on that route. I think there are other ways to get to WI from the city.

Day 1: Illinois Prairie Path Elgin Spur to Elgin, then Fox River Trail north to Algonquin, IL. At this point, we lost the trail and ended up biking on roads all the way to McHenry, IL, where we got on another trail leading to Genoa City, WI. Trail ends there. Took roads ("Old 12", I think, is the road you want to take) to Big Foot Beach State Park in Lake Geneva. (~70 miles)

Day 2: Made our way southwest to Union, IL to KOA campground outside of town. No specific route to share. Just used maps and took what looked like country roads. (~50 miles)

Day 3: Union, IL to along roads to Dundee area, then on the Fox River Trail to Prairie path back to Wheaton area. Again no route to share. We just made our way with a map and I can't remember what roads we took. (~50 miles)

Hope this helps. On my next trip I'm going to keep a note pad handy to keep track of routes, etc.