The third Doctor's era contains many six parters so I'm afraid you're goiong to have to get used to the longer stories if you keep watching his stories. And that season only had four stories in it and some 25 weeks to fill, so they went with one four parter and three seven parters.
is pretty good; it's the best of Pertwee's first season, in my opinion.
Agreed. But it could also be an episode shorter. They were trying desperately to keep costs down that first Pertwee season to help offset the transition to color, and it shows with the padding. I like The Ambassadors of Death but it really should be a four-parter.
They do get better at not simply padding the six-parters as the Pertwee era goes on. The Mind of Evil is fantastic, for instance.