Michael Palin admits in "The Pythons" that by the end of season three, Cleese was bored, and they were all running out of good material. He and Terry Jones had come up with the "Cycling Tour" idea as a project for the two of them to do by themselves, but when the Pythons were coming short of new ideas, they pitched it to the others, who added their own bits.
This has always been my least favorite episode of the first three seasons, an experiment that failed dismally. Mr. Pither is a character that might have been funny in a 3-minute sketch, but not as the anchor for an entire story. The same goes for Mr. Gulliver, whose changes of personality are funny for about five seconds apiece. Chapman's Chinese ambassador is an overlong and embarrassing segment; the firing squad scene would have made a good brief sketch but goes on too long. Doing a funny accent is not an anchor for an entire segment.
What made the "normal" episodes work so well was the frequent change of story line, the different personalities of the writing teams, and the craziness of Gilliam's animations (which are drastically underused here). When I first saw this episode almost half a century ago I was disappointed; watching it again today left me with the same feeling.