What Japan does in the north really depends on what Russia does.
If they pile up all 18 infantry on Amur, then do like Wittmann said, pound them good and quick. Except for the 6 guys in Mongolia, that basically opens up all the Russian far east territories for Japan to walk into over the next few rounds. Almost like free IPCs for Japan.
If Russia withdraws all troops to head for Moscow, which I have seen happen, that leaves you free to use most of those troops in Manchuria and Korea in China or other places you might need extra troops. Just leave a few there on the border. Then after 3 rounds or so, when the Russians are too far west to hinder you, walk some troops up and start taking Soviet territories. Of course, you will have to deal with the 6 Mongolian/Russian troops but that shouldn’t be too difficult.
If Russia only withdraws some of the far eastern troops, like say he leaves 6-9 infantry in Amur and heads the rest toward Moscow, that usually means he wants to guard his eastern territories but isn’t looking to be aggressive toward Japan unless a blatant opportunity comes up (like Japan withdrawing all troops from Korea or Manchuria and leaving them totally open). In that case, as Japan, I would simply leave the troops there on Manchuria and Korea and consider that area a “non-combat zone”, meaning there is simply no fighting going on there. You could possibly take a couple of the troops to use elsewhere, but you don’t want to weaken youself too much or Russia might take advantage. Yeah, it sucks leaving so many guys just sitting there, but you have to keep Russia honest. Plus, at least you start with those guys and any new stuff can go to your campaigns elsewhere.