Russian and German fought during winter WW2 ! less than in summer, but when the soil is freezed you can move and fight. Anven during the rasputitsa.It's unclear how long will it takes Russia to bring all these new conscripts to the battle front. Russia's biggest weakness in the war was it's lack of infantry. With Russi can bring 100000 reservists in to Ukraine in a month and a half, the war will become a stalemate and winter will set in.
Russia will then have whole winter to train up the conscripts.
Ukrainian (and US) top generals are aware of this. They will not let the time to Putin to train the reservists.
And even so, these troops will fight with small moral.