That only works if the German army heads for Stalingrad and Moscow. Vilna to Pskov is about 250 miles. The German army can cover the distance in about 1 month at their historical rate of advance in September 1915, putting them in Pskov before the end of October 1915. Hence no land to trade for time. The Russians hold Pskov or else they die. And the broken Russian armies cannot stop the German 11th Army, cream of the Heer in October 1915.