The period of a century or two was not that big for a Master Ranker like Nina.
The idea of staying apart from Eren for such a long period was indeed a bit difficult for Nina to digest. However, she also treated it as one of the motivations she needed to progress in her Ranking Status.
She was also fascinated by the prospect of exploring realms other than Anfang. She felt like she had again become a beginner Ranker and was preparing herself for her days as an adventurer ahead.
Seeing Eren progress in his Rank had lit a fire inside Nina's heart as well. It was as if she wanted to impress Eren after he was gone by giving her everything to progress ahead in her path as a Ranker.
Eren smiled at Nina after seeing how understanding she was. From his side of the table, he let his hand approach hers before holding it firmly.