The Kaplan videos are excellent, and do a good job of painting a full picture of medical biochemistry, as well as honing in on high-yield concepts. They are lengthy, though, as you already know. One option is to watch them at 1.5x or 2x speed. Another is to spot-review just concepts you feel unsure about.
Realistically, most of what you need to know for boards is in FA. The real challenge is not memorizing the material, but understanding the concepts and being able to apply them when presented with exam scenarios. Most questions on the step will be in the context of biochemical disorders, so try to acquire a stereotypical picture of how each condition in FA will present - at what age, with what signs/symptoms, and what the blood tests will show. This is where question banks are invaluable, as they paint that picture for you. Every now and again the step questions will simply ask a direct question about some specific enzyme or pathway.
There was an earlier discussion on the matter in
this thread. Some suggestions included FA Express, Lippincott Biochem, etc. Regardless of what book or video you choose to use, use FA as homebase, supplement its bare-bones description with more vivid details where needed, and try to anticipate a question on every biochem disorder in it.