Jonjo Shelvey admitted he "wasn't fit enough" when he first joined Newcastle United.
Shelvey, an influential player in the Championship last season, has hit form in recent months.
However, the midfielder struggled for form and fitness when he signed for Newcastle from Swansea City in January 2016.
“When I first signed, I had a few good games and a few bad ones and to be honest, I wasn’t fit enough,” said the 26-year-old.
“But now I have that sharpness back to my game, and am doing it in the Premier League, which is the highest level you can do it, so that’s what I’m pleased about.”
Shelvey found himself on the bench in the final few weeks of the 2015-16 season. That was the catalyst for the England international, who worked on his fitness during the summer ahead of the club's Championship campaign.
Shelvey hasn't looked back since, and he insists that he wants to stay at United for "many years".
“When Rafa (Benitez) came to the club, he chopped and changed and I found myself out of the team," Shelvey told NUFC TV.
“But I went away that summer and got myself properly fit and came back with a totally different attitude in terms of trying to learn from him.
“He signed me for Liverpool, but I never actually got to work with him.
“It was more a case of wanting to stay here and get this club back to the Premier League, because they paid a lot of money.
“So it was about returning the favour, and hopefully I can stay here for many years now.”
Newcastle manager Rafa Benitez made it clear that Shelvey was not for sale in January amid interest from West Ham United.