Barca have 40 points from 16 matches ahead of second-placed Real, who have only won twice in their last eight games in all competitions. Flick, however, warned against complacency.
"It's a long road ahead," he told reporters. "It's December, and we've got a lot to play for.
"The important thing is to look at ourselves. There are things we can do better. The right attitude is not to be happy with what we've already got. We have to keep going.
"And the end of the season is what will put you in your place. That's where you'll have earned what you've got."
Barca welcome 15th-placed Osasuna on Saturday full of confidence, buoyed by the return of winger Raphinha and midfielder Pedri.
Both played crucial roles in Tuesday's come-from-behind 2-1 Champions League home win over Eintracht Frankfurt, which marked Barca's fourth successive victory across all competitions.
While results have been encouraging, Flick continues to face questions about Barca's goalkeeping situation.
Marc-Andre ter Stegen, 33, was named in the squad for the Frankfurt game but remained on the bench.
He has yet to feature this season, having suffered a back injury in July. He subsequently became embroiled in a dispute with Barca, who briefly stripped him of the captaincy when he declined to sign a long-term medical leave that would have eased the club's financial pressures under LaLiga rules.
In his absence, 24-year-old Joan Garcia - signed from local rivals Espanyol in the close-season — has established himself as first choice, making 13 appearances in all competitions.
Flick was unequivocal when asked about Garcia's status: "Joan is number one. I'm not going to talk about the number two or three goalkeeper. Joan plays, we believe in him, and I have no intention of replacing Joan. And he has done very well."
Real manager Xabi Alonso faces mounting pressure ahead of Sunday's visit to Alaves after his side's drop in form.
Flick declined to be drawn on his rivals' difficulties.
"I'm not worried about Real Madrid, only about ourselves and our current situation," the German said.
