I can't speak for climbing just the five miles to the top. I have done the entire event, and the year I did it, the weather was great. However, it was a good 20 degrees cooler at the top than when we left Spartanburg. I've heard tales of riders starting out with temps in the 70s and by the time they reached the top, it was in the 40s with sleet or some other type of winter-like precipitation.
AOMM, in my opinion, is one of those events that you can only train for by going up and riding the actual roads. In my area (west central Georgia) there are plenty opportunities for climbing, but nothing that will prepare you for the 27 miles from Marion to the top of Mount Mitchell. With the exception of a mile stretch of downhill around the 95 mile mark, it's all climbing. I climbed for the better part of 3.5 hours in that 27 miles. It's not real steep, but there are some gradients that reach 6 or 7%.
As far as descending it, you are not allowed to during the actual event, but I bet it would be fun.