De Marcos took the captaincy this season after Iker Muniain's departure.
The midfielder said: "The youngsters have taken a step forward. Together we are helping to ensure that there are players in good dynamics.
"I feel good, it's true that I know myself well enough, I know when I'm well and the manager knows it too. I'm good enough to play in that position.
"Once I make the decision I consider that it was the right direction. I take into account and evaluate everything from the previous season and I decide to continue for another season. At the moment, I am feeling well.
"I have believed in previous years that it could be the last and in the end I make another decision. Each time I see myself closer to the end. It is not the time to evaluate it."