In the book Aikido, 1958, K. Ueshiba, under the guidance of Morihei Ueshiba, in the section called "Basic Knowledge" are two points (out of nine) dedicated to ki flow. Even though the concepts were presented as "basic knowledge" and fundamental to the practice of Aikido, it is acknowledged by the author that they are not easy concepts to grasp or explain.
I'd be fairly cautious about assuming how much guidance there actually was from Morihei.
Kisshomaru talks quite a bit about writing this book in "Aikido Ichiro".
He does state that he asked for his father's guidance, but he also states that he wrote the entire text himself, that it was edited extensively by a third party, and that he changed many particulars of his father's explanations in order to make things more understandable to the general populace.
He never once mentions actually receiving any guidance from his father.
According to Kisshomaru his father's response when he talked to him about writing the book was "well, do what you like", and his father's response upon seeing the finished work was "hey, great, it's got my picture in it".
It even goes on to describe a child who is incapable of lifting more than 50 pounds being able to "bewilder" someone capable of lifting 500 pounds. From this description I find it easy to infer two things; Ueshiba recognized and utilized ki as something other than muscular strength; that anyone is capable of this use of ki, not just shihan.
This is one of Koichi Tohei's classic examples, and of course his influence at the time the book was written and published can't be underestimated.
In the later "Aikido no Kokoro" ("Spirit of Aikido"), Kisshomaru speaks about Ki in some detail, describes it in classical Chinese terms, and cites the Chinese origin of the entire concept. He doesn't relate it to being muscular, or to the lack of muscle. He goes on to relate it to modern science.