The 24-year-old penned a six-year deal with the Magpies after his contract came to an end with Gus Poyet's men at the end of June, becoming the first player in 16 years to make the switch between the Tyne-Wear rivals.
He told the club's official website: "There's been a lot of ball work among doing the running as well. It's been good, it's nice to be back. The lads have been great and very welcoming.
"It's good to have a lot of games to play - that's where you're going to get your fitness and your match sharpness and start getting a feel for the game again, which you maybe lose a touch from being off for so long."
Newcastle begin their pre-season campaign on Tuesday against Oldham Athletic before touring New Zealand at the end of the month.