Yeah, but FP was building on the already-established association of "flying saucers" with spaceships. The concept of flying saucers had been an ongoing fad since 1947, and flying saucers began showing up in film in the 1950 movie
The Flying Saucer, followed by
The Day the Earth Stood Still and
The Thing from Another World in '51,
Invaders from Mars in '53,
This Island Earth in '55, and probably others.
Earth vs. the Flying Saucers came out just months after FP. So it was firmly entrenched in an existing design trend.
By the way, I said earlier that every film & TV spaceship I knew of before the
Enterprise was either a saucer or a rocket, but I've just found one exception: the
alien ship from It Came from Outer Space, which was a geodesic-ish sphere with a tailfin.
No, his original idea was that the whole ship would land. From p. 10 of the original series pitch: "The Cruiser will stay in space orbit, will rarely land on a planet. Landings are made with a small (and transportable) recon rocket vehicle." Remember, it wasn't Roddenberry who designed the ship's appearance, it was Pato Guzman and (mostly) Matt Jefferies. They gradually arrived at the saucer-and-cylinders design after dozens of prototypes, some with saucer elements and some without.
http://forgottentrek.com/designing-the-first-enterprise/
I really wish people would stop giving Gene Roddenberry, a writer, credit for the work of his production designers. He guided the design process by approving or rejecting their concepts, yes, but he didn't do it all himself.