Gerad Dean briefly described the route for the Summit Trail (starting at around 6700' elevation):
During a recent trip to Hawaii (Big Island) I wanted to see if I could complete the Mauna Loa Summit Trail from car->summit->car in a day. The trail, from trailhead to summit, is about 19.5 miles in length and boasts approximately 7000’ elevation gain. It is a long, steady, and generally gradual climb. The trail, marked by ahu (rock cairns), is nearly entirely above tree line and makes its way across a stark landscape of recent lava flows. There is essentially no vegetation, making the entire trail exposed to the sun with very few shaded breaks. It also lacks any reliable source of fresh water, so I would need to be entirely self-sufficient on this day.
Another Variation is "Sea to Summit" (ascent only), about 54 miles.
Just did this route, its epic and brutal! You slayed it.