The point is to make the German PAY to take these places. Leningrad is the more critical point – as it allows FTRs from UK to land, which you can bring into the Red air force. If Germany takes Leningrad on turn one, you can easily re-take. Then station plenty INF there, and remember: even a UK FTR sitting there will help defend, hitting Germans at 4-or-less rolls. Leningrad can be the site of see-saw battles, because the German knows you need it (or Vyborg) to land Allied FTRs coming to your aid. But he generally doesn’t devote sufficient troops to HOLD it, at least in the first few turns.
KEY: Keep Germans 2-territories-away from Moscow as long as possible.
Let’s say he takes Stalingrad, and can blitz through Turkestan to hit Moscow, avoiding your main force in Belorussia. You must: place 1 or 2 INF in Turkestan as “speed bumps” to gain TIME.