In my opinion, the best way to get better at climbing is to climb, pure and simple. Whether you do that on your own, through training rides or organized rides, doesn’t matter as much. What matters is that you put in the work.
The Freewheelers have a series of official training rides, but they are mostly self-managed by the cycling community. Richard White is especially instrumental in putting them together. A good way to keep up on them is via the eRide (free) and Freewheeler (members) mailing list, where all the details will be posted.
The core group of people who put this together are fellow mountain goats, the same ones that do a lot of the more difficult climbing rides in the southeast. Many if them have done Mitchell several times, and they know what they’re talking about. The groups are usually divided up into A and B riders, and occasionally there will be a C group for the beginning riders.
The Saturday ride is usually the long ride for the week. They will get progressively more difficult in the weeks until Mitchell.
There are two difficult rides they’ll do that are worth mentioning. One ride will start from Table Rock State Park, will head into Rosman, and up Highway 215 through Balsam Grove to the Blue Ridge Parkway, then back through Brevard. It’ll be around 1000 miles with 8,000 feet of climbing.
Another 100 mile/8,000 foot ride is from Landrum to Tryon, up the Saluda Grade, before heading back via Caesar’s Head Mountain.
Many of their adventures will occasion on the Tryon/Saluda area, and throughout the series, they’ll hit most of the signature climbs in the area. They’ll also mix it up flat and fast rides to make sure people get a variety of training options.
If you want to go it alone, there are options. First of all, note that climbing and descending can be dangerous. You should know your own ability, and be as familiar with the area as possible. Plan for your rest stops so that you don’t run out of fuel. You can find routes at Spartanburg Cycling and SteepClimbs.