I am ready for it, having found that the 18,800' of climbing in the Devil Mountain Double was not all that tough.
With 20,300' feet of climbing in 198 miles with elevations up to 10,000', the Alta Alpina 8-Pass Challenge is the toughest double century I know of, including possible extreme temperature swings (could range from freezing to 100°, same day, depending on weather).
This is what the organizers have to say:
Excellent! My preferred stuff. But for this sort of endurance stuff, I greatly prefer Perpetuem over HEED (and I can’t stand lemon lime flavor of any brand). Perpetuem has some protein content, important to reducing the breakdown of muscle tissue for fuel.
I’ll be going 5 days in advance so that I am fully acclimated by the day of the event. Although I need only 1-2 days to adapt to altitude to a meaningful degree, 4-5 days means I can just totally forget about it as a factor— though power is always degraded by about 15% by 10,000', just because there is much less oxygen per full lung intake.
The main risk in high Sierra riding is severe dehydration and hyponatremia (loss of elecrolytes which can make you puky and end up with an IV in your arm, or worse). The use of a product like Hammer Endurolytes is essential. See also Dehydration and Fluid Intake for Endurance Efforts.