Ukraine readies counteroffensive - What will it take to win back territory?

Russian missile and drone strikes have hit several Ukrainian cities overnight, killing at least 12 civilians. A residential building was hit in Uman, 200 kilometers south of the capital Kyiv. Authorities also reported deaths in the eastern city of Dnipro, where they say a woman and a young child were killed.
Ukraine and the western world have been waiting for months for Ukraine to begin a counteroffiensive against entrenched Russian forces in the eastern part of the country. But the challenges for Ukrainian troops are immense - as they take on a much bigger military force. What will it take to win back territory in Ukraine and why has it taken so long to prepare? What could be the key challenges for Ukrainian soldiers during the coming offensive?