We did the trek to Luxmore Hut and back to Te Anau as a day-walk.
The track is very good - almost a road for this portion.
It is easy to get the water taxi across to Brod Bay (and back if you want) to cut out the walk around the lake so it is not too long for a day trip.
It is quite a steep ascent - but a well cut zig-zag path that decreases the slope. Sheltered in the trees for most of the way, you emerge into the elements above the tree line before reaching Luxmore Hut - a good spot for lunch.
TIP: If you go, take a head torch so you can explore Luxmore Cave just near the hut.
Great views from up the top.