I remember doing the Cascade Saddle route about 5 years ago. When checking the LP 'Tramping in NZ', it said that the part between Aspiring hut and Dart hut is quite long. So I started my walk from the carpark at the end of the matukituki valley (took a minivan from Wanaka), walked past the Aspiring hut and camped on the treeline halfway through the climb to the first ridge (pylon).
From there, I had a wonderful view on Mt Aspiring, turning pink around sunset... it's one of my best memories of a year spent tramping all around NZ. I hope to one day return and climb Aspiring.
If fit, it should be perfectly possible to hike from Aspiring to Dart hut in one day (taking an early start) without a problem.
From Dart hut, I continued along either Rees or Dart valley (don't remember which) and took a minivan to Queenstown and back to Wanaka.
Anyway, it's one of the nicest tramps I did there. So, have fun (and good weather!).