Halfway to Anywhere

In his book of the same title, G. Harry Stine has this to say: "The title of this book, Halfway to Anywhere, was suggested by Tim Kyger, currently a staff member of the Senate Committee on Commerce, Science, and Transportation in Washington. It sums up where we are today with respect to real space transportation for everyone and comes from the statement--correct from the viewpoint of the energy and thus the rocket propellant required--I first heard in private, personal conversation with the late author, space advocate, and contemporary philosopher Robert A. Heinlein in 1950:

"Get to low-earth orbit and you're halfway to anywhere in the solar system.""

Now, here's my question: Did Heinlein ever use this in a story or novel? I swear I remember reading it in one of the Future History stories, or maybe one of the YA novels, but I haven't found it so far in my search. Anybody remember this as well?

(Halfway to Anywhere: Achieving America's Destiny in Space was written by G. Harry Stine in 1996. It was published by M. Evans & Company, N.Y. There's also a companion book by Stine, Living in Space, published by the same company.)