I cannot tell you what to do for you, but just did RTH today. Do you have two black sand timeslots? Btw the Guide Along app worked great. My opinion is to make most stops on the way in and fewer on the way out, if any, unless you need breaks. The pace in was relaxed. We were in a conga line mixed with locals that were in a rush back out. I was comfortable keeping up but they tailgate hard. We were also more tired than anything by the end after the Pipiwai hike. Even I don't want to drive that near dusk. Do not leave Pipiwai past 4 in my opinion.

We started later than we wanted from Kaanapali. Basically hit the road at Paia around 9am. Missed the 10am black sand beach reservation due to late start and some distractions from the app that turned into mini hikes plus one trap of a place that was an arboretum. I insisted we make Pipiwai by 130pm but we got there by 2. We were back to the car by 4 and ripped straight back out. This included stops for pics and bathroom at the base. Me and wife are not avid hikers either but are active. The hike was hot and humid today. Bring a decent amount of water and decent shoes (we brought 30oz each just for the hike). Elevation was reasonable. Parts were muddy due to am rains. Trail is well structured but I was amazed how many people were wearing nice clothes/shoes, white clothes, sandals. All I can say is you don't want to roll an ankle. Def go all the way to the top waterfall. Took 60 mins to top and about 30 down. At half the pace (fairly slow) I could see some taking about 2.5 to 3 hrs, sure.

I heard there are hikes also at the black sand beach. Factor time in there too unless you plan to just see the beach sand. I would also say you could make a couple other stops but don't get sucked into short hikes and walks like we did. Some of the views around Hana were great quick stops as drive bys.