When I was preparing for my first Mitchell, I found myself getting psyched out about the route. At that point, despite having some decent fitness, I was not confident I would be able to complete the event. Since the challenge with Mitchell is as much psychological as it is physical, it is a great benefit to have an idea what to expect.
This visual map gives a lot of terrific details about the route, rest stops, landmarks, and so on.
I found Paul Thomas' section-by-section summary of the route to be invaluable. If you have not ridden before, I highly recommend you read each section and prepare accordingly.
Part One - Intro and the start
Part Two - Pea Ridge Hill
Part Three - Dangerous Curve
Part Four - Bill's Mountain
Part Five - Tom Johnson Campground/Marion
Part Six - Highway 80
Part Seven - Blue Ridge Parkway
Part Eight - Mount Mitchell State Park and The End
In a nutshell, the riding will begin easily, with fast and smooth highways. The further the route takes you, the more challenging it gets. Bill's Mountain is a tough climb, and a good test of how you are doing that day. As you approach Marion, you will experience more and more rolling hills, many of which will be steep.
After leaving Marion, you'll have a few more miles of easy riding before the real climbing begins. Many consider the climb up Highway 80 to the Parkway to be the toughest part of the day. The Parkway is more manageable, with a consistent grade and even a short downhill section, but it goes on a long while. The State Park is no cakewalk, with some steep sections that especially hurt after all the miles you've put on your legs.
The general rule of thumb is that it will take you just about as long to get from Spartanburg to Marion, as it will from Marion to Mitchell.