I rode up Mt Mitchell today, and it was a very nice ride but I'm not sure I get why many people think it's so difficult. Admittedly, I rode up from Asheville (70 miles roundtrip), so I'm willing to concede the possibility that it might be tougher coming from Marion.
However, Mt Mitchell itself is not very steep, particularly compared to the grades of Hogpen, and Brasstown.
The sustained climb is definitely longer, but the average grade for the 4 miles up Mt Mitchell felt like maybe 5%.
I'm not saying it's easy; it definitely gave me a very good workout, but it's not the epic beast that it's sometimes made out to be.