FATMAP difficulty grade
The Cass-Lagoon track is a popular 2-3 day tramping track that's also a very pleasant trail run.
It follows well-marked trails connecting two sheltered valleys, passing over two alpine saddles, with forest, alpine scrub and stream sections, numerous huts, and increasingly open and wild running in Cass stream near the end.
Being east of the divide, and with no major river crossing issues, it can be run for much of the year. The track heads up through beech forest and then breaks out to give superb views nearer to Lagoon saddle, before descending into the Harper River system.
It's a pleasant run down through here - the trail in this section is part of the Te Araroa Trail, however you leave this when Hamilton Creek joins in - cross a left over a bridge and head a short distance up Hamilton Creek to the spacious Hamilton Hut. From there, the trail grinds its way up valley, which becomes increasingly steep as it leaves Hamilton Creek (this is the one place you won't be running!) then eases o before Cass Saddle.
From here it's a delightful descent on fast trails down to the Cass river, with some river crossing and running on faint trails across river stones to follow.
Eventually a marker guides you over a stopbank onto a 4WD track to follow an obvious long straight line of pine trees that leads to your bike (or second car).