Kursk Oblast is 7-8 hours drive from Moscow. Rylsk is just over an hours drive from Kursk nuclear power plant. Kursk is 2 hours drive north from Belgorod, which is a staging area for the Russian army. The populations of these places aren't high, somewhere between 250-350k people. With any other country, there wouldn't be the resources to push forward. But with Russia, there's no civilian defence, no army reserves, and lots of empty space to burn through.