Are you going to look Matthiessen state park?http://dnr.state.il.us/lands/landmgt/parks/r1/mttindex.htm
IF your going up that way I know of a MTB trail along the trail getting into Senica
right along the School right before crossing the road.
Im not sure of the condition, As I got a flat right there,
and discovered it trying to find a place to camp out with a Friend
at night, In the morning decided to fix my bike while waiting for breakfast.
(but I think the condition is bad worth a look though,
but couldn't ride it anyways because I had too much gear/weight.)
I've been Wondering about Matthiessen mtb trails for a while, haven't heard much about them (other then on here.)