The Grant Woods Forest Preserve has trailheads off Route 132 (Grand Avenue) west of Fairfield Road, and also off Monaville Road west of Fairfield. There are several loops north of Monaville Road and another loop south of Monaville. An underpass gets you to the other side of Monaville Road. The trail is crushed limestone and is in excellent condition. A map is at the end of this post.
Though difficult to discern, this is a very hilly area (for Illinois) and confers a good workout to bikers. The southern section of the trail passes through an Illinois Nature Preserve before ending at Rollins Road where it connects with the Chain O' Lakes Bike Path, a paved trail which runs downhill alongside Rollins Road to the town of Fox Lake.
Fall is an especially beautiful time to bike or hike this area, as seen in the next two photos. You also see a bit of the uphill in the next photo.