What should I plan for on a sea-to-summit ride on Mauna Kea, Hawaii? I've driven up to the visitor station but not up to the summit - what are the road conditions like beyond the visitor station, where can you refill water, and should I prepare for anything beyond a leg crushing day?

I'll be starting on Kona (west) side, in September, and am expecting about 6-7 hours of straight climbing.

Here's about all I know so far;


  Mauna Kea, Hawaii, HI 
  
  This mostly paved climb is perhaps the largest
  rideable climb in the world.  The first 34 miles are paved, then 5
  miles often loose cinder gravel, and then 3 final miles of pavement. 
  The first 28 miles up Saddle road from Hilo gains only 6600 feet at
  modest grade, tapering off to almost flat in the saddle between Mauna
  Loa and Mauna Kea.  Turning off onto Mauna Kea Access Road is a real
  shocker, as you are now faced with sustained 17% grades at high
  altitudes.  The Fiets index for this climb is off the charts at nearly
  30.  It’s a combination of net gain and high altitude driving the high rating.  Like Haleakala, views are too good to describe with words. 
  Many of the world’s most powerful telescopes are at this summit.


ref: http://www.northeastcycling.com/Favorite_Climbs.html

Thanks
I think this long post by Martin Hoff also indicated by jurgemaister in a comment tells you everything you need. You can find some more info in the comments on individual attempts people did of the segment on strava. The main takeaways I got from the article are:


You need two bikes (a road bike and a mountainbike) or a dirt-road suitable bike.
You optimally have a support vehicle driving along with you for food/drinks/extra clothing/emergencies/the way down/nice pictures/the second bike.
If support isn't an option you need to pack for a really long hard day in both winter and summer climates. Even if you are very fit. Use panniers mounted on your bike, a backpack carrying enough stuff will probably break you.
Mauna Keau tops out at over 4K above sea level. How fit will you be up there? How used are you to high altitudes? Altitude sickness might be an issue, strenuous activity will certainly be very hard.
You need low gearing for the steepest part. Install a mountainbike crank-set.
If you take one bike you probably want knobbly tires on it.