The 32-year-old Pole, who has limited movement in his right arm, drove 115 laps in a 2012 car for the Renault team at a test in Valencia on Tuesday.
"I'm proud with what I achieved but also it shows what I've lost," he said.
"I don't know what the future will bring but I ran with good pace and consistently in difficult conditions."
Kubica's F1 career was cut short following a crash in a rally car in February 2011 as he was preparing for his second season with Renault.
He was severely injured when a crash barrier pierced the car, causing multiple arm and leg fractures and partially severing his right arm.
Kubica, who won the 2008 Canadian Grand Prix for BMW Sauber and was considered one of the fastest and most talented drivers in the sport, competed in rallying from 2013-16 and has begun to explore a comeback to circuit racing.
His comments, made in a statement by the Renault team, did not address whether he thought he was capable of returning to F1.
So far, this has not been possible because the limited movement in his right arm had meant he was not able to turn a steering wheel sufficiently in the tight confines of an F1 cockpit to be able to drive on all the tracks on the calendar.
Robert Kubica said he had "mixed feelings" about his performance after his first test in a Formula 1 car since injury curtailed his career in 2011.