We've not done the trip you're considering but my husband and I have done a lot of road riding on the Big Island. If not for the trip into Waipio Valley, I'd recommend a road bike because you could ride so much faster - you could do one day to Hilo; one day back. We've always rented from a shop in Kona - it's on the water side of the highway a little south of Costco - and they have some nice, new, high-end road bike rentals. We pack our own seats and pedals. There are a lot of good riders there to hook up with, likely because of the Iron Man. The pavement is great. Bring plenty of water and plan your riding so you're not out during the heat of the day.
I'm curious what your planned route is. A detour through Hawi is nice. Did you find a shop that has locations in Hilo and Kona so you can do a one-way rental?

[This message has been edited by letiole (edited 01-20-2003).]