More than 30,000 people competed in the Boston Marathon this year. A guy wearing bib No. 4848 finished 4,155th in a time of 3 hours, 9 minutes and 7 seconds.

Not bad for a 43-year-old fellow running in his first major marathon.

“I spent four months getting ready,” said seven-time NASCAR Cup champion Jimmie Johnson, who is also a triathlete. “I’d much rather cycle or swim. Running’s third in line on the list.”

He’s not ready to quit his day job, even though it’s not going so well these days. Johnson and the No. 48 Chevrolet (hence the number on his marathon bib) are 16th in the standings, with no top-five finishes and six top-10s.

Going into next Sunday’s Toyota/Save Mart 350 at Sonoma Raceway — which is celebrating its 50th anniversary — Johnson has a winless streak of 74 races in the NASCAR Cup Series. His last win was June 4, 2017, at Dover, Del.

It won’t be easy for Johnson to snap the streak at Sonoma. He has just one win in 17 starts there — and that (his only road-course win) came in 2010 after leader Marcos Ambrose stalled his engine while trying to conserve fuel under a caution.

Johnson won the series title in 2016,but was 10th the next year and a career-worst 14th in 2018 — a season in which he was winless for the first time. It was a bitter pill not only for Johnson but for Hendrick Motorsports.

The rocky season led Johnson and crew chief Chad Knaus to part company after an often spectacular 17-year run. Knaus took over as crew chief for 2018 Rookie of the Year William Byron and the No. 24 Chevrolet for Hendrick. Byron is 14th in the standings, three places ahead of Johnson, halfway through the season. Johnson now works with Kevin Meendering, who was in the Xfinity Series last year.

In a phone interview with The Chronicle, Johnson said there were “cracks” in his relationship with Knaus going back as far as 2015. He wouldn’t get into any specifics in discussing the breakup.

“It was just time for us to do something different,” he said. “We weren’t bringing the best out of one another, or the performance level we needed.”

Johnson indicated there have been some growing pains for his new team with Meendering. “Of course, I want to get back to victory lane and wish that I was there already,” he said. “But we’re still growing as a team, and that’s where it’s at.”

Until 2015, he said, he and Knaus “were always able to get mad (at each other) and work harder and make ourselves better and faster. In 2015, with all the pushing and all the work and all the effort, we could not put speed in our cars. That put a lot of pressure on our relationship, and I think that led to fractures at that point.”

NASCAR Cup Series Toyota/Save Mart 350 Where: Sonoma Raceway When: Friday-Sunday Schedule:Friday — Practices at 11:35 a.m. and 2:35 p.m.; Saturday — Q ualifying, 12:10 p.m.; Sunday — Toyota/Save Mart 350, noon Circuit: 12-turn, 2.52-mile road course Number of laps: 90 Last year’s winner: Martin Truex Jr. Tickets: $30-$140 single day/$95-$180 weekend. Parking is free. Kids 12 and under are free Friday and Saturday; $10 on Sunday. Info: 800-870-RACE, SonomaRaceway.com/NASCAR

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They turned things around temporarily in 2016, when they won the championship.

“But when we got back into 2017,” Johnson said, “it felt a lot like 2015 — all this work and effort and not being where we needed to be. It tore down what our relationship was. In 2018, we decided we needed to make a change” midway through the season.

Many observers wonder when Johnson will follow other drivers of his generation, most notably former Hendrick teammate Jeff Gordon and Tony Stewart, into retirement. Yet Johnson insists he’s a better driver now than he was in his championship 2016 season.

“Every day, every week, every month, every year, I try to find ways to grow and be stronger and better and learn from mistakes,” he said.

A couple of days after the interview, Johnson made a key mistake on pit road that contributed to a 15th-place finish in Michigan on Monday.

His chances of getting that eighth Cup championship, which would break his tie with NASCAR icons Richard Petty and Dale Earnhardt, are getting slimmer each year.

He has talked with drivers of the generation before his, people like Rusty Wallace, Mark Martin and Dale Jarrett, about their own retirements.

“I’ve seen guys who were definitely ready when it was time and then walked away peacefully,” he said. “I watched others, like Rusty, that picked the date and said, ‘This is when it’s going to happen.’ That date showed up and he wasn’t ready. He still wanted to race, but he had everything in motion, and it was time to stop.

“Through all these conversations, I’m just trying to be aware of my desire to compete. The day that wanes or the day that fades, that’s when it’s time to step down. That has not happened yet.”

Tom FitzGerald is a San Francisco Chronicle staff writer. Email: tfitzgerald@sfchronicle.com Twitter: @tomgfitzgerald