In my opinion, the best way to train for an endurance climbing event is simple -- climb more and ride more!
The Freewheelers have already started their official training rides. The easiest way to hear about them is either through the Freewheelers mailing list (for members), or The Assaults Facebook and Twitter feeds. If you live in the area, I recommend trying to join them for as many of these as you can accommodate.
Once the weather warms up, and the sun shines a little later, there will be plenty of other, organized rides. I've been fortunate enough to participate in quite a few of them over the last couple of years, and would suggest a handful to prepare for the Assault on Mount Mitchell.
Tour de Lure - 4/6/2013
The 71-mile option out of Marion, NC is possibly the best training ride for Mitchell. After a climb up Stone Mountain, and passing by Chimney Rock and Lake Lure, you'll come back to Marion via mostly the same route as the Assault on Marion. My total time on this ride last year gave me a reasonable expectation on how I would perform for the first half of the Assault.
2013 Italian Road Bike Festival - 4/6/2013
This is a good alternate training ride in North Georgia, probably a good option for those who live south of the Spartanburg/Greenville area.
Assault on the Carolinas - 4/13/2013
This is a metric century out of Brevard, NC. It boasts some nice climbs, including Walnut Hollow, Highway 178 to the Continental Divide, and Caesar's Head Mountain. The event is extremely well organized, and a lot of fun.
Issaqueena's Last Ride - 4/27/2013
This century ride out of Walhalla, SC, has only a couple major climbs, but don't let that deceive you. It is mostly rolling hills, up and down for the entire day, with a lot of steep pitches. This is a difficult ride that will prepare you for the climbing and saddle time of Mitchell.
Tour de Cashiers - 5/4/2013
A number of friends consider this ride to be at least on par, or maybe even more difficult than AoMM. It is easily one of the most challenging century rides in the southeast. There are some daunting climbs, including Tilley Creek and Walnut Creek. In the past two years, I've found that my time on this ride was a good indication of my time for Mitchell. Plus, the timing works out as you'll begin tapering for Mitchell shortly afterward.
If you would suggest any others, please share them in the comments. Depending on where you live, I would also recommend trying to find a couple local century rides, whether officially organized or just with a group. In my opinion, the more opportunities for long rides, the better prepared you'll be for Mitchell.