No problem. You'll find that even if you go slow and stop a lot along the way, that you can get to Oheo Gulch in about 4 hours. Make sure to stop and relax at Oheo Gulch for a while though. But the return trip tends to be a lot faster since you will have done all your stops already. You'll be able to get from Oheo Gulch to Lahaina in 3-4 hours depending on how many people are on the route that day.
Buy this book it is a must if going to Maui.
http://www.wizardpub.com/maui/maui.html Maui Revealed.