Add Heading HereI recently hiked Mauna Loa via both the Mauna Loa trail and the observatory trail. Neither is without its challenges.
The Mauna Loa trail involved some red tape. With the Merrie Monarch festival in full swing, the NPS closed off the road to the trailhead to prevent festivalgoers from picking flowers. The only way to get access to the road was to get a backcountry permit, which meant pretending that we were going to overnight on the mountain. It was an extra step, but ultimately proved to be no obstacle. Definitely check with the visitors center in Volcanoes NP before you head up. Just be prepared for their weirdly patronizing attitude towards, well, everyone. My hand literally got slapped by a ranger for scrolling a computer screen (incidentally, that same ranger told us we'd encounter a fence on the trail and to make sure to close the gate to keep out feral cats, when in fact the fence keeps out feral goats).
The trail itself isn't bad. I've seen people complain of boredom and a rough slog over lava. The trail is long, some 20+ miles one way. All horizontal distances on Mauna Loa are bigger than anything you're used to, because it is the world's largest mountain - more like a very high hill. We turned around after 10-11 miles at the 11,000-foot level, and it still made for a 9+ hour day. But the reward for that is an all-encompassing landscape that constantly varies and is as peaceful and quiet as it is alien and inhospitable. The altitude was noticeable, but not bad (although not everyone will have that experience).
The walking surface wasn't bad at all. I'd feared rough, uneven lava would be punishing, but was pleasantly surprised. Most of the footing is solid and totally manageable. The early portions of the trail are even smooth.
The observatory trail, despite requiring no special permit, is even more painful to get to. The road in from Saddle Road is as bad as any I've ever driven: pitted, steep, pitching, twisty, one-lane and interminable. Like a mosquito buzzing in your ear, it seems like an active tormentor, not some inanimate object you chose to travel.
Once again, the reward is an incomprehensibly massive landscape that swallows you whole and some great high-altitude conditioning. We hiked to the trail junction at the pit - it didn't seem worth 2+ miles just to gain 200 feet in elevation from that point on.
The views from both sides were incredible. Mountains, clouds, ocean, lava - truly an epic place. The rock formations are unlike anything I've seen anywhere else - even in other volcanic settings - and we even saw patches of snow.
I didn't try the Ainapo Trail, as much as it would have provided the best shot at a true one-day summit. What information I could glean (and it's not easy to come by) is that it requires 2+ hours of driving on a 4x4 track to get to the trailhead. Too much time to take out of a vacation, even for someone for whom long slogs in thin air are a pastime.