Please do that as a land trip when you possibly can. It is a wonderful trip. Spend as much time on Maui with a rental car as you can. We spent 5 days there several years ago and loved every minute of it.
Check out the tours that take you to Hana and beyond to the pools (7 sacred pools). They take the scary road (back of Haleakela) back to Lahiana and it's much shorter than the 10-12 hours if you go and come back the same way. Plus I think you'll see more because one of you won't have to drive.
Road to Hana, done on your own or a tour is best done in a leisurely manner, stopping to see the ever changing landscapes and sights. Full round trip unless you are just going to drive and not get out 7 or 8 times to enjoy what there is to see will take 10 hours minimum.
We did the Road to Hana in May and from the Maui airport in Kahalui, it took us about 8 hours total. We did not get out at every stop along with all the other people but we did stop at several waterfalls and high points along the way. It is a beautiful drive and one that I would highly recommend doing. Make sure to watch for hidden waterfalls on the corners (we found an absolutely gorgeous one hidden behind some foliage that was huge!)
In '01 we did 2 cruises to Hawaii and both times I drove the road to Hana. We overnited so we had no time problems. There are variables as to when you actually get your car and begin your adventure. You must also then look at the time you must return it and get a shuttle to the pier. When those times are known, do the division and that is when you must turn around to head back. The roads are great and what cannot be determined is the traffic and the # of times you stop to explore. Even though you might not get to Hana, you will experience a portion of this amazing rain forest. I'm booked in Jan. '06 and will do what I just told you. On our 2nd trip we pulled over at a spot where the highway begins and there was an amazing view from a cliff overlooking parasurfers down below and rainstorms off over the ocean with rainbows. It was an amazing sight to see so you never know what you might experience. On one trip we stopped at Wal Mart and bought lunch meats, rolls, etc. and we had a picnic at one of the overlooks. That was time consuming so you might get some hoagies before you begin the trip. There are no restaurants along the way so take your own food and drinks.