I'd check out:
http://www.harpethbikeclub.com/Routes/Routes.aspx for some routes that you might be able to piece together or add to, to get to 100 miles. Also here's the Harpeth River Ride 100 route:
http://www.harpethbikeclub.com/Rides...e/Century.aspx. This is pretty hilly for these parts with some steep hills thrown in for good measure. This ride might start too far south for you, so if you really need to start in Nashville I'd look for routes that start out of the Model Airplane Field at Edwin Warner Park and combine some of the routes you'll find on the Harpeth Bike Club page.