Just finished this, I'd been watching an episode per day. I had expected this to be another story, whose primary flaw was that it had been stretched for budget reasons.
It is too long, but not badly stretched as some others (Pertwee era has the worst offenders). The shoestring budget does show, lots of running about between three sets which are shot from a limited number of angles. In best Who tradition, the set decorators do their best with a dozen shower curtains and some metalwork they found next to the bins.
None of this matters, as of course Troughton is wonderful. As long as he's given enough material, and is reacting well with the cast it all works.