I know Haleakala well.
I have climbed parts of it.
One of the things that you have to be prepared for is the change in weather conditions as you climb. It can be hot, humid and windy in Paia, and can be very cold at the top.
The ascent while, only about 5.5% on average, is relentless. There are almost no downhill segments, and there are some 10-15% sections, including at the very end. It is an endurance ride. It has been done in under 3 hours, but I would expect that you should plan for 5-6 hours of uphill riding.
Remember the air thins out above 8000 feet. That makes wind resistance less, but there is less oxygen in the air. Some people get altitude sickness. If it happens to you, (like your nails turn blue), descend immediately. I mean immediately. An ambulance could take 45 minutes to get to you, and might take just as long to get you to the hospital.
If you descend after you climb, you will have to deal with traffic on the roads. In many places there is no bicycle path (breakdown lane), or any wide road. The road is not up to modern standards. If there has been a storm, and the DPW has not cleared the breakdown lane, you will be riding in the road. While this is true about the ascent, the speed, and the twists in the road make the descent more dangerous. A lot of the drivers do not like bicycle riders, especially if they interfere with their commuting. Remember it is an active roadway, not a bicycle path. Depending on the trade winds, you descend into a turn and all of a sudden you are riding into a wind that is pushing you off your bicycle.
The other thing you have to plan for is refueling. Once you hit Crater Road, there is no food or water until you hit the Park Visitor Center. There are not many places to get food or water after you leave Makawao.
If you have any other questions, feel free to ask.