Look at a map and see what is furthest from a highway, but technically accessible by some sort of old road or trail. Look at a mineral rights map and find the most remote gold rush era mining claims.
You won’t see people often in these places because your standard 4x4 lacks the fuel range to get there- gasoline engines have very poor range at low speeds. You need something capable but also fuel efficient- usually something with a small diesel engine, which are not very available in the US market. More remote places also tend to have a lot of cumulative water damage to trails so are very technical- requiring a lot of patience (e.g. stacking rocks for hours) and a lot of driving skill.
A fun thing about Baja is that often these super remote fuel stations end up being closed or out of fuel, so you can't realistically do it without having the supplies and fuel for at least 2x what it looks like you should need.
That's basically where I'm getting my "two days" from- the time from one fuel station to the next with about ~8 hours of actual travel per day.
