Sparta / Elroy Classic is a 90 mile lightly trafficked loop trail located near Sparta, Wisconsin. The trail is primarily used for road biking and is accessible from March until November.
Helpful tidbit: bring a flashlight. You'll need it for the long tunnels where you're supposed to walk your bike.
Road this as a kid as a three day trip.
Went with a group of guys back in 2000. The trail was great and the scenery was awe-inspiring. Great for biking but I wouldn't recommend hiking due to all the mountain bikers. Did about thirty miles of the trail over the weekend.