a) Spend 20PP on rail capacity. Yes, it's expensive, but it will allow you to move your northern armies south next turn, whereas it takes a whole three turns to recruit a new unit.
b) Buy a garrison with your remaining 10 PP.
c) If you can afford it, repair that garrison on 8hp.
That's the easy decisions... now the hard ones.
The basic choice you have here is between sitting tight and digging deep, and giving ground to try to shorten your line. I would be tempted to do the latter I think. Use the Fortress at Brest-Litovsk as an anchor. Fall back with the infantry and garrison nearest Warsaw (try to form a line directly to the north of Brest-Litovsk if you can). I would also fall back 1 hex with the cavalry unit SW of Brest-Litovsk.
With your northern armies, garrison Vilna with the garrison and start entrenching (it may come to that). Stay put and rest the infantry, you'll be able to rail them south next turn with your expanded rail capacity.
There's no right and wrong answer here. The downside of falling back will be that you'll be abandoning entrenched positions.
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