Back in September a friend and I decided to traverse the entire Yatsugatake (八ヶ岳) massif from south to north in 2 days. We set out lightweight from Tokyo on the first Azusa Limited Express train on a Sunday morning, and began running from Kaiooizumi station (one stop before Kiyosato) at about 10am.
Day 1 took us up into the alpine and over Gongendake (2715m), Akadake (2899m and the highest point of the massif), Yokodake (2829m), and Iodake (2760m), and down to the Neishi-sanso hut, where we spent the night. The next day we continued north over Neishidake (2603m), Higashi-Tengudake and all summits between there and the final major peak of Tateshina (2530m). Finally we descended southwest from the summit to the trailhead at Tateshina-tozanguchi, eventually finishing at the hot springs of Tateshina-onsen.
The total running time for the whole traverse was 18 hours, with 7.5 hours on Day 1 and 10.5 hours on Day 2. We were fairly unlucky with the weather, and had to contend with strong winds, thick cloud and rain for much of the time, which left the trails and rocks very slippery particularly on the second day. Drier more stable weather might have further reduced the necessary time for the traverse, but ultimately that's just speculation.
A lot of the route is not really runnable in the usual trail running idiom, but for a fit party it is certainly possible to cover ground quickly. This route is normally hiked in 4 days, but if you are fit and confident in what you are doing then it's perfectly possible to complete in 2 days. The next challenge would be to do it in one continuous stretch without the night in the hut...
Anyway, here's a video I made of the trip. If you're interested in going for it, I hope it gives you a flavour of the traverse: