We did a very similar ride about 20 years ago on our first trip to Maui. There are a number of things that you should know about it now as the excursion is really in three parts and the duration from pick up to drop off is basically the whole day (7 am to 4 pm).
Part one is from the pick up at the hotel, a nice feature, with a fairly long drive to the business office to sign waivers, get helmets and windbreakers, and then a drive up to the summit of Mount Haleakala. We were given a chance to walk around the observation point (at 10,000 feet above sea level) and then a van ride part way back down the mountain to get to the bicycle part. It seemed as if this first part took between 4 and 5 hours (no biking). The guides, Tom and Billy, got mixed reviews from our group. There was non stop talking from them about everything you can imagine dealing with Maui, the mountain, the history, etc. After a while it got to be too much in my opinion.
The second part of the trip was the bike ride which starts outside the national park at about 6,500 feet and descends to Paia at sea level. This tour was "guided" meaning that we followed a tour leader down the mountain and we were followed by the van which helped block/control traffic as we descended. The problem is that the trip down is at the pace of the slowest and most inexperienced bike rider and I can almost guarantee there will be someone in your group that is literally afraid of bicycles (People say they can ride but they really can't).. My hands were numb from gripping the brakes constantly on the way down! Not a lot of fun when you might like to "let her rip" a little where it is safe to do so. 20 years ago the bikes started in the summit parking lot (10,000 feet) but that is no longer allowed by the Park Service so the riding part is significantly shorter now.
The final part of the trip was "lunch on your own" in Paia. Paia is a quaint older town maybe leftover from the 60's.It was very hot the day we went so it was a bit uncomfortable.Lunch options (now after 2 pm) were few. Then back on the van and the return to the hotel.
I debated whether to opt for the guided tour or the "self-guided" tour (this company offered both). I II ever do it again I will do self guided. The downside to self guided is there is the potential to get lost as there are a few turns onto different roads on the way down, and you have to trust that the cars coming down the mountain will respect your rights on the bicycle.On that point, all the drivers we encountered coming down were courteous and gave us a wide berth.
The other factor is the weather, particularly at the summit. On our visit it was cool and the beginning of the ride required using the windbreaker provided. Shorts were fine even from the start. I heard that it can be very cold on top of the mountain any time of the year so layering clothes is important. Since it gets quite warm at the bottom, a backpack to store your layers when they are no longer needed would be helpful.
All in all, the bicycling part has gotten shorter and the riding around in the van, etc has gotten longer. Some thought the "talking" by the guides was over the top (too much), others seemed to enjoy it. Personal preference.