Think about it like this…
Every game Russia defends against Germany. It will counterattack here and there but the main emphasis is usually to stockpile infantry on the front lines and defend. Germany, in turn, endlessly sends units to attack and the result is usually a drawn out conflict until an ally helps.
Instead, Germany should play it as such…
Take the territories that will give it the national objective bonus (the eastern front territories) and then just stockpile infantry to defend them. Play against Russia the same way they typically play against Germany. Play defensively on this front. This may hold off any Russian attack since you’ll have a few extra IPCs to spare. Now focus all your might on UK. Sure, you may not be able to take the capital, but it diverts resources from the UK. These are vital resources that they need to build IC in India and Africa if they want a good chance to hold the territories. The UK will have to make a choice. Either defend the capital or defend Africa/India. The India/Africa region is more difficult for UK to defend than it is for Germany/Japan to attack. Several more senior posters have mentioned the ease at which Egypt can be taken by second turn (at worst) and India (second turn probable). If Germany is focusing 80% of its resources at UK, I don’t think UK can afford to split resources between capital, India, and Africa.
Even if you don’t or can’t take the capital, this is the opening that the Axis may need to wipe out Asia and Africa. Once you have control of both of those, it’s practically a lock for the Axis based on massive economic might.