On the contrary, Kerouac had been preparing notes, text, key phrases, etc., for years - ever since his late-40's cross-country travels with Neal Cassady. Here is an actual April 1949 entry from one of his journals, courtesy of Douglas Brinkley's Windblown World: The Journals of Jack Kerouac 1947-1954 (2004).
WED. 27--Started "On the Road" with a brief 500-wd. stint of 2, 3 hours duration, in the small hours of the morning. I find that I am "hotter" than ever--tho on closer examination afterwards I figure I may only be over pleased with words, and not structurally sound yet (after a long layoff). My interest in work is at a high pitch. My aim is to have much of "Road" done, if not all, by the time T & C is published next winter. I quit school today so I can do nothing but write.--Now I want to expand the original 500 words which, in the heat of work, 'discovered' an important opening unity.
Brinkley includes photocopies of certain pages from Jack's journals, the above entry being one of them.
Jack, I love you, man, but this is one myth that needed to be busted.
The truth is out there.*
*Gratuitous pop culture reference.