Does it make sense to get an LLM in preparation for "law school?"
No. It does not make sense to do a LLM before a JD.

The length of the program is not the relevant thing to consider. 

The standard course is to do a JD -- which is a first degree in law -- before a doing a LLM to gain specialized knowledge. Usually a JD or equivalent is required for admission into an LLM program. 

I understand the confusion since usually a doctorate (JD) comes after a masters (LLM). Note that, outside the US, the first degree in law is often called a bachelor of laws (LLB), which makes more sense.