There is a nice grassy plain from Poland all the way through to the Caucasus (with the Urals as a somewhat permeable barrier). Armies on the move love grassy plains.
If your objective is to protect Moscow, you need to control that grassy plain.
Edit: ref: Prisoners of Geography: Ten Maps That Tell You Everything You Need To Know About Global Politics by Tim Marshall.
"Geography is destiny" can be a useful lens through which to view global politics.
That would be the Pripyat Marshes? That is notoriously inhospitable terrain for armies.
They're not really a big obstacle, given the two land invasions that came from the West in the 20th century.
Both unsuccessful. But I note your observation that the Pripyat is not as large as my crap geography led me to think.
The Polish invasion in 1920 was pretty effective too until it wasn't, same for Napoleon, etc. Point being its a big corridor for large troop movements.