@Der:
For example, do you leave one infantry in each bordering territory like this to foil blitzing, or is there something better to do? Do you just do this all the way back to Moscow?

do not leave your infantry for blockers
germany will take those territories with overwhelming forces. ussr will lose the infantry and have a 1 in 3 chance at taking an infantry from germany.
it is even better for ussr if germany blitzes. if they blitz through the territory deeper into ussr territory you can then kill their tanks in the counter attacks. if they blitz the territory and move back to leave the newly captured territory empty you can take it back easily on your turn. a good german player will not do this as they are going to keep pushing their stacks towards moscow.