This isn't the first Ukrainian offensive. They've mounted several offensives that ended in total disaster. Just before this one, they launched an offensive in Kherson and failed miserably. The Russians are retreating since they never really held much territory in the north. They didn't even take Kharkov.
This offensive is mostly a propaganda campaign to secure more western funding. It's just the initial push, once it's over the Russians will regroup and retake their positions. Ukraine can't sustain this for long.