I rode the trail to Hillsboro while riding the 400. It's very scenic; however, some sections of the trail were a little overgrown. Also, there was some bridge repair done this summer, and when I was on the trail, there was still a sign saying that the bridge was out. Fortunately, they must have forgotten to remove the sign, because the bridge was fine and the trail to Hillsboro was unimpeded.
Once you get to Hillsboro and you feel like doing some hillclimbing, you can get on HW 33 and head to Wildcat Mountain State Park and Ontario, WI. (where you can rent a canoe to enjoy the scenic Kickapoo River). From Ontario, you can bike north to either Norwalk or Wilton and get on the Elroy-Sparta trail. Heading south on the E-S, you will eventually rejoin the 400 and return to the Hillsboro trail turn off.
There are some major hills between Hillsboro and Ontario, but the road from Ontario to Wilton is easier.