It is common belief that ck is in a sense closer to the knowledge, the interests & the capacity of a medical school graduate. And, to some extent, this is true and works for many people.
However, when preparing for Step 1, not only is a candidate forced to revise basic concepts from the scratch, but also gets used to a certain way of working things out. This very type of exam in not exclusively a matter of knowledge; it demands a multidisciplinary approach to most questions and multiple higher functions of the brain are recruited in order to retrieve the correct answer. In other words, it's not the amount of textbooks you skim through (which is enormous nevertheless), but how you manage this amount (and time) and how you incorporate this knowledge in your personal theoretical & clinical problem-solving habits.
To conclude with, Step 1 equips the potential exam taker both with solid knowledge on basic science, but also with knowledge about the USMLE exams themselves. Many concepts that you come across Step 1 will certainly reappear in Step 2, with more clinical correlation. If you have established a solid background concerning mechanisms throughout Step 1, then studying for Step 2 will be much easier (not piece of cake, though!). Vice verse in not necessarily true...
Good luck with whichever Step you chose to take first (because the final choice will & should be absolutely yours) and be strong & patient. You will need your mesolimbic & mesocortical reward system now more than any other time before!!!