Johnson wore number 4848 as a tip of the hat to his car number – 48 – and finished his 40- to 44-year-old age group in 641st place.
The 43-year-old tweeted afterward that it “was the most challenging thing I’ve ever done.”
Johnson was hoping to break 3 hours, but said he was pleased with the result.
“I wanted to race it, so I went out with a heart rate and time in mind. I came up a little short, but still to do that well, still to be that low in the threes, I’m really happy with it,” NASCAR.com reported him saying.
Johnson had previously completed a 70.3-mile triathlon and has his sights set on entering the 140.6-mile Ironman Triathlon one day. He often responds to alleged fans on Twitter who tell him that he should concentrate on driving, given the decline in his on-track performance in recent years.
He couldn't care less about them on Monday as American distance running great Meb Keflezighi placed a finisher’s medal around his neck.
"This was so cool," he tweeted.