Such a fun and unique trail. The first few miles are in mixed forest, scrub forest, and then occasional bushes. After that you are out in the open and there isn't much vegetation at all. You get to see a whole mix of lava geography - olivine sand (my favorite), red clay/sand, black pahoehoe, red and black aa. There are several cinder cones and the trail gets you pretty close to them. The summit caldera is massive and hard to put into words. Much more scenic/varied, and much less crowded than Mauna Kea.
This was a burly route and I was not quite up to the challenge. The summit is at 13.6k feet, and you spend a lot of miles above 13k vs. most summit pushes which are quick up and downs - Mauna Loa is massive and it's a gradual climb. I didn't get much acclimation ahead of time and developed a headache a few hours in, which gradually got worse as the run progressed. Didn't puke but felt close to it, especially towards the end. I guess if you want you could spend a few days at altitude here (maybe staying at the cabins) but that didn't line up with my schedule.
Ran unsupported with 3L of tailwind and water. Was able to fill up at the catchment at the Red Hill Cabin (note this requires water treatment), I don't think there is any other option for water on the course. Perhaps in the winter months you can find some ice/snow near the summit and melt it.
One big note for future runners - the lava rocks here are very sharp and they will shred your shoes (and then your feet). Pick something with a very firm/rugged sole and prepare for the shoes to be worthless after. 40 miles of this stuff did more wear on my shoes than 600 miles of "normal trail".