The most direct route is Jolon Road, which is the way I went to get to the National Forest this past fall. I ended up doing an extra loop in the FHL base because Mission Road seems to go 3 directions and I missed my turn, but got straightened out in less than 5 minutes. If you take a closeup map of the FHL roads you won't get lost. Speeding is a big no-no on the base, so you may as well go straight through.
The day I was there, nobody checked ID, hunting weapons, or anything else. I hear that is normal when there isn't any training exercises going on. You don't need to go into secure areas to get to Nacimiento Road.
The road is really twisty. I made my wife carsick in a Geo Metro, and she doesn't get carsick ever. There are probably some potholes, as it is secluded area. There also might be gravel in the road from long vehicles clipping the dirt in sharp corners. You may even see some wild pigs and deer.