They don't need to, are you forgetting they ammassed 150,000 on the border befor the invasion?
They still have 140,000 troops left, it's a significant advantage and they will chip away
Don't Russia also have trained reserves, who number almost 2 million people who have served as conscripts in the Russian Armed Forces for at least one year within the past five.
Add to that the 350,000 personnel in the National Guard service and a further 20 million in a different military reserve category (those who served more than five years ago).
There is a massive reserve force to call on should they need to.
Ok yes they might not be as well trained, but sheer mass of numbers, will be a force to reckon with.