Robbie Russell (3) will stick with D.C. United for one last match on Saturday before retiring to pursue a career in medicine. (Jeff Zelevansky/GETTY IMAGES)

Robbie Russell’s greatest fear, as he departs pro soccer after a dozen years and opens life’s next chapter, is remembering what it’s like to be a student.

“I’ve got to take notes on a laptop,” D.C. United’s veteran defender said. “I’m used to a three-ring notebook. Everything’s changed. I’ve got to relearn how to go to school.”

Russell, 33, is not returning to the classroom to complete a long-delayed undergraduate degree or to enroll in a business course during the offseason.

He plans to become a doctor.

On Wednesday, 21 / 2 months into the MLS season, Russell announced he is retiring and will enroll in Georgetown University’s Post-baccalaureate Pre-Medical Certificate Program, an 18-month pathway to medical school.

“I am following up on a second dream,” he said, citing orthopedics and emergency medicine as his top interests. “It’s exciting but very daunting. I know I am going to be the oldest guy in the class, the weird old guy in the back of the class raising his hand. Everyone is going to look and think, ‘Who is that, the teaching assistant?’ ”

Russell, who has appeared in just one of 10 matches this year, will remain with the team through Sunday’s match against Sporting Kansas City at RFK Stadium, then turn his attention to Georgetown orientation two weeks later — and years of studying and training.

“It’s tough leaving with the way things have gone with the team,” he said of United’s 1-8-1 record, the worst in MLS. “I feel like I should help, but I have to do what is best. I’ll be rooting for them all season.”

Russell, who earned a sociology degree at Duke, had considered deferring enrollment into the Georgetown program until the fall. But that would have bumped his timetable for taking the Medical College Admission Test back a full year. He wasn’t going to play much this season anyway, so he approached United officials last month about retiring.

“We didn’t expect it this soon, but it’s a tremendous opportunity and time for him to take the next step,” said United General Manager Dave Kasper, who can fill the roster opening anytime and apply Russell’s $75,000 salary toward future acquisitions. Frankie Simek, a former U.S. national team defender, began a tryout this week.

Russell’s career took him to Iceland, Norway, Denmark and Salt Lake City, where he played 31 / 2 seasons and celebrated an MLS championship in 2009. His penalty kick clinched the tiebreaker against David Beckham and the Los Angeles Galaxy.

Eager to reunite with his wife, a Washington-based attorney, Russell was traded to United before the 2012 season. He made 19 regular season appearances (16 starts) but missed a chunk of the year with a foot injury. In the playoffs, he assisted on Nick DeLeon’s late goal in a series-clinching victory at New York.

After the season, when United declined to exercise a contract option and offered a reduced salary, Russell recognized one career was ending and a new one could begin.

Throughout his time in Europe, he had a curiosity — but not the time — to pursue medical interests.

“If I wanted to do it, I had to go all out,” he said. “At the time I couldn’t go all out, so I thought, what could I do in the meantime to get my brain back into it?”

In his second season with Real Salt Lake, he interned with the club’s physician, Andrew Cooper. For more than a year, after practice and on off-days, he followed the orthopedist on rounds and witnessed numerous surgeries.

In Washington, he has done the same with United physicians James Gilbert and Victor Ibrahim.

Both wrote him letters of recommendation for the Georgetown program. Both also treated his foot injury last season.

Doctors are “always thinking of the next thing: ‘How do I improve this? How do I make this better?’ ” Russell said. “Always changing, always learning something new and different is exciting to me. It gives me the fire to learn.”

Russell has enjoyed an exciting life. He was born in Ghana, where his American father was working in international aid and his Ghanaian mother grew up. They lived in Sri Lanka before settling in Amherst, Mass. While in Europe, Robbie read Icelandic literature and learned Norwegian and Danish. He can trace his paternal roots to the founders of the Mormon church.

Becoming a physician after a long career in athletics is a great leap.

However, Russell’s father-in-law, Leo Berkenbile, an ER doctor in Glendale, Calif., instilled the confidence to pursue it.

“From Day One he said, ‘You can do this,’ ” Russell said. “If he hadn’t been there to say that, with all of the other doubts swirling through my head, I don’t know if it would’ve happened.”

Russell said he hopes to attend Georgetown or George Washington medical school. Someday, maybe, he will return to United — repairing ankle and knee injuries instead of suffering them.

“That would be pretty cool,” he said. “I’ve got to stay friendly with these guys.”