Given the POD offered, the Germans have plenty of food and supplies - the war with France and Britain is over, they're not under blockade, a quick victory means Italy and Romania (and less importantly, Greece) never joined the Entente. It means that Austria-Hungary never had her military hollowed out by the Brusilov Offensive, so Germany isn't lugging around an ally that's just dead weight. It means the Ottomans are free to open up a meaningful second front with Russia; even if all they do is stare at the Russians and look menacing, Russia still needs to deploy troops.
Germany and Austria-Hungary are absolutely capable of taking Moscow and Petrograd in this scenario, if somehow the Russian Empire hasn't sued for peace long before either city is besieged.