Definitely ride the John Muir Trails and the connector trail between it and the Emma Carlin System. The outer loop at John Muir with the connector is close to 25 miles. With Emma it's 30mi and a long day on the bike.


Blue Mounds State Park is also a popular (difficult) trail in the South WI area.
I rode CamRock a few years ago and it was pretty good. Some cool tech features and rocky terrain. MTB Project CamRock page. It's about 45 mins Northwest of the Muir trails.