We loved the Hana highway and were lucky to do this special tour with a van for 10 persons.
Okay its not relaxing, but its worth the time and the exertions and you can see many parts of the island.
Unfortunately I dont remember how long it took, but we had more time, because we had 2 weeks for 4 islands.
However, you will enjoy everything on Maui.
We have been there in february 6 years ago and the most important thing for us was watching the humpback whales close to Maui. Thats the reason we came to Hawaii and of corse our 25th wedding anniversary.