The name of the game for Classic specific training is to go long. Speed matters less than length. Training as you would for an ultramarathon is probably the most appropriate but anything that lasts for more than 6 hours is going to help.
Long runs are probably the best but hikes, bike rides and skiing are also good. You should keep moving all the time. If you get tired, slow down but don't stop. Work up to a 12 hour day, then 15. Then do two 15 hour days back to back. On the Classic you'll be carrying at least 20 lbs in a pack so don't forget to add some weight in a small pack in your training.
The leaders and winners of the Classic usually combine a fast walk and running for the first several hours of the race. Try mixing a jog with a fast walk while wearing a 20 lb pack for two hours at the beginning of a 15 hour day. If you can do that, you're in decent Classic shape.
Obviously volumes can be written about training alone so I'll post more about it later.