I would not get too worried about that ride. My guess is that if you cycle frequently then you will be just fine.

I have done that ride on our tandem. We did not use the granny. I think you will be totally fine on a single bike if you have been riding some before the trip. It is not that steep. It is however fairly long.

It is a beautiful ride. I am sure you will enjoy it. I am jealous.

Don't forget to ride around the island as well. The complete Hana road is fun. Stock up on water before you head back on the desert side as there are no services on that side. We ran out of water. It is a must do ride as well.