Spanish clubs began training in groups of 10 players on Monday, in line with health protocols, as La Liga took another step towards the planned resumption of the season next month.
"I was missing training, my team-mates, playing football and all that," De Jong said in an interview on the club website.
"Two or three months apart were a long time. It is always nice to meet up again after a separation for holidays or whatever, but it's normally just three weeks or so and this was almost three months."
Teams in the top two divisions in Spain returned to their training grounds earlier this month, with players initially working individually as part of La Liga's staggered programme.
"I don't think I'm ready to play a game yet. We still need a few weeks of training. First individually, then in small groups and then as a whole team. We'll be ready to play again in a few weeks' time," said De Jong.
"I think it was good to start by training separately because everyone worked at their own pace and could see how their fitness was before working with the rest of the group. I think everything felt good enough! It was a good build-up."
La Liga chief Javier Tebas is hopeful matches will return from June 12 but admitted the exact timing will depend on the health authorities in Spain and the trajectory of the virus.