It is kind of going to depend on how early spring hits so you can ride. I would say try for the 45.
But I want to talk you into a different ride. The best ride I have ever done anywhere is the Tour of Lakes which is going to be June 1. There are two choices. A ride that will be 35 to 40 miles or a ride that will be 60 to 70 miles, depending on what route they pick. The long ride usually has at least one significant hill. The food is amazing. There will be many rest stops and you should be able to do the 35 to 40 ride by June 1. I did the long route last spring. It was my first 100k. The route goes around many lakes and through the woods. It will be in the Brainerd lakes area. The scenery is beautiful. It likely will be rolling hills but nothing tough on the shorter route.
http://www.paulbunyancyclists.com/To...f%20Lakes.html
But I understand if you want to do the Tour de Cure. That would be fun too!