This was the most pleasant ride of my stay in Hawaii. I did it early in the trip but only just now got around to writing a lot of this stuff down.
I started in Waikoloa, went up the Queen K to Kawaihae and then on to Hawi, on the north side of the island and the turnaround point for the Ironman. It is about 2 hours of mostly flat terrain to Hawi.
In Hawi I turned right on Hwy 250 and started riding up the Kohala Mountains towards Waimea. It turns out that I turned a bit earlier than I should have, and did a couple miles of 10% gradient that I could have avoided if I had turned right later on Kinnersley. After that, though, the climbing is moderate, and the day I rode, the views were outstanding. Once you get some altitude, it becomes very green. Basically the "green line" is about 1500 feet, where the rain becomes consistent as the clouds move up over the mountains. You are now in ranch country. Waimea is the center of the ranching industry on the Big Island, and home to the Parker Ranch, which is the second largest ranch in the US after the King Ranch in Texas.
The road surface is good and traffic is light. The climb tops out around 3700ft after 13 miles, for an average gradient of 4.4%.....very pleasant. From the top there is a nice descent to Waimea, and then the ripping descent from Waimea back to the coast.
There is no water between Hawi and Waimea, so fill your bottles in Hawi (there is a market behind the post office at Hwy 250). Fill them again in Waimea (turn left and go 2 miles to the town proper where the shops are), because you are still about an hour from home, and by now it is probably ass hot.