I would like to offer a few reasons as to why Mito herself may not have actively defended her native land from destruction.
First, the culture. The period that Mito lived in was the warring states period, based no doubt on the warring states period in Japanese history. During this time it was common for marriages to seal alliances between factions (for example, Oda Nobunaga marrying his sister into the Azai clan.) While these kinds of marriages in theory promised lack of aggression and help in times of great need, this was not always the case (Nobunaga later destroyed the Azai clan despite his sister marrying into it because there was an even greater conflict of interest.) In any case, once a woman was sent to another clan to be married into it, she became the responsibility and property of that clan. Mito, through marrying Hashirama, became a konohagakure ninja and would not be considered an uzugakure ninja anymore. Her responsibilities now lay within Konoha, especially as the wife of its leader. She had a new family and role in the village. She could only return to uzugakure if Hashirama SENT HER BACK. I doubt, after Hashirama's death, that the shrewd Tobirama would send Mito back (for military purposes) nor would Sarutobi send her back (for her own protection).
Next let us consider the timeline. During Hashirama's reign, uzugakure was still an active village and supposedly influential village. At the very least they were able to survive long enough as a village to send Kushina to the leaf. This means that both Sarutobi's generation and Jiraiya's generation went from childhood to adulthood between the time of Mito arriving and Kushina arriving. In other words, while Mito may have seen the slow destruction of uzugakure, she may have simply been too old to be of much use in a battle (remember that she was the same age as Hashirama, the same person that taught a very young Sarutobi and was the GRANDFATHER of Tsunade.) Being a jinchuuriki is not of much use when the container holding the beast is brittle. Mito lived long, but that does not mean her body stopped aging. Rather, her old age could potentially last very long.
What do you think?