OK, try this one, it should be about your mileage range and starts and ends in Kona. It's a ride I did south to Captain Cook/Kealakekua and back:
Starting at the waterfront in Kona, take Alii to a left turn at Walua Rd. Follow this to where it joins with Kuakini Hwy (this is the IM route in town, you'll be going slightly uphill to the turnaround), and then get on the main highway (Hawaii Belt Rd/Mamalahoa). One block later, you'll come to a stoplight. Turn left (onto Lako St).
About 200 meters up the hill, you'll see a gate to a bike path to your right. This is one of the big unpublished secrets of the Big Island! Get on this bike path. It empties onto a narrow side street that takes you past lots of houses, and it roughly parallels the main highway for a few miles just about to the top of the grade. It will make a few turns, but it's a gentle climb that avoids all the traffic, and it's pretty easy to follow. I think there's finally a sign that says "dead end" or something, but that's where you turn right and get back on the main highway.
Follow the main highway for several miles until you get into the village of Captain Cook. You'll soon get to a road that veer off to the right (State Hwy 160). Things get pretty quiet from this point. Continue down the hill (as I recall) until you get to the intersection with Middle Keei Road to the left, and take that to the intersection with Painted Church Road a kilometer (half mile) later.
Painted Church Rd ends at a wide highway. Turn right and descend to the beach. The park on your left is Pu'uhonua O Honaunau, the Place of Refuge. You can stop here, and/or continue on the flat, narrow road to Kealakekua Bay, where Captain Cook was killed. From here, you can take the long, twisting climb up Napoopoo Road back to the main highway. You'll see the Middle Keei intersection, and at this point you'll be backtracking. I stopped at a small restaurant mid-way up this climb for lunch, but there are lots of places to eat along the main highway. There are also a few coffee plantations where you can sample their (very expensive) Kona coffee.
Check it out on google maps. The bike path is not there, even with "bicycling" turned on, that's how secret this is! Good luck.
Luis