Welcome from the Grafton area! The Oak Leaf and Ozaukee Interurban trails are great, although they can be heavily populated on weekends. A good ride to train for is the Trek 100 in June. It's one of the best supported rides I have done, and by far my favorite organized ride. You'll probably find organized rides have a Metric century (~ 62 miles) rather than a half century, but if you can do 50 miles, you can do 62.
Welcome to the addiction!