If you can tolerate chilly nights, I would say the North Country Trail should be good to go by early-mid April. In 2011, I did two s24o's- April 29 -> 30 and May 20 -> 21 and the trail was in awesome shape, albeit a few downed trees. I started from the 56th Street trailhead both times and headed north for an out and back. Singletrack galore... Very few people and cars. You can get info/gpx/etc. here: http://transmichigan.blogspot.com/
As far as routes down south, this one looks pretty interesting, but alas, there's lots of road and rail trail:
Main route page: http://www.adventurecycling.org/routes/ ... ?pg=detail
Event page: http://alleghenymountainsloop.blogspot. ... -2012.html
Not that you're looking to do an 'event', but both links have good info. The AML you could probably pack light and two-day ride a shorter loop with an overnight in a town/motel. The same with the NCT as well.
If you need a waypoint file (water, food, camping, motels, etc.) for the NCT, let me know and I'll post one up.
There's' also the Trans-Georgia route, but I don't have any experience or knowledge of it, so you'll have to do your own homework. Cite: http://www.firstworks.com/tnga/