Brief report:
Left my hotel in Kona just past 6:00am. Arrived at the Onizuka Visitor Center (9,200') by 7:45am. Started hiking from there at 8:15am on the Humu`ula Trail (Mauna Kea Trail). Took a quick side trip to visit Lake Wai`au then traveled clockwise to check out all the telescopes (from the CalTech sub-millimeter telescope to the UK infrared telescope). Made the summit by 12:05pm. Stayed there for about half an hour. Started going down at 12:35pm. I tried hitch-hiking but no one gave me a ride, so I had to walk back down the trail. Got to my rental car by 2:15pm (6 hrs car-to-car). Back at my hotel's pool around 4:00pm. Fun, easy day on top of Hawaii.
Additional info about climbing Mauna Kea can be found here.