"I picked up swimming late," he said. "I didn't start swimming year round until after my sophomore year."
Johnson was about 16 when he really got his feet wet in the sport. Michael Phelps was breaking national records when he was 14.
Thus, Johnson has plenty of gas left for the long haul, should he wish to make that journey.
Johnson qualified for his first FINA World Championships by taking second in the men's 200-meter breaststroke at the Phillips 66 National Championships and World Team Trials in June. That earned him a trip to Barcelona, Spain, where he swam his first race on Thursday while hoping to reach Friday's finals.
Unfortunately for Johnson, that didn't happen. While he moved on from the prelims with a 2:11.64 (15th out of 16 qualifiers), his 2:10.79 in the semifinals landed him 12th and out of the finals.
Despite that disappointment, Johnson's swim career is nonetheless on the upswing. His personal best of 2:10.09 from the U.S. nationals left him ranked No. 9 in the world and No. 2 in the USA this season. Perhaps he's just scratching his potential?
"That's hard to say. I'm not going to put any limits on myself," Johnson said. "To be honest, I was fairly disappointed with the swim at World Championship trials. Obviously, second was good enough, but I don't think I swam the race well. And, I don't think I swam like I normally swim the race. There were a lot of things I could do better and work on . . . there's definitely room to improve upon that swim."
Just the fact Johnson has come this far and to this stage is quite remarkable, given his start in the sport.
"If you look at my (long-course) times coming out of high school, I was a nobody," said Johnson, a graduate of Garfield High in Seattle. "My freshman year (at Stanford) I swam 2:17, which used to be pretty good. Then I went through some struggles as an undergrad."
Due to knee and technique problems plus the fact Stanford was loaded with great breaststrokers, Johnson actually dropped the event and became a sprint freestyler.
Clearly, Johnson was not your typical college swim star.
"No, not at all," he said. "And if you're not a star coming out of college, you don't keep swimming."
"My last year in college (2009), after doing freestyle all year, I did one more 200 breast and went 2:15."
After taking a year off from competition, during which he did some training and played club water polo, Johnson realized that competing would be more fun than just training. He returned with a 2:13.29 in 2010-11, followed by a 2:11.47 at the 2012 Olympic Trials (sixth) and a 2:10.87 following the Trials at the U.S. Open.
His first big meet back was the 2010 national championships in Irvine.
"That was the meet I actually went best times after not really swimming at all," he said. "That's when I went, 'wow.' It piqued my curiosity."
At the U.S. nationals at Stanford in 2011, Johnson qualified for the 'A' finals in both breaststroke events for the first time.
"And that's when I really had my career rejuvenated, when I realized I had a shot at making an Olympic team, which I didn't do. I didn't make that goal, but I knew I had a shot at making other teams in the future."
When Johnson beat current World Championship teammate Kevin Cordes at the 2012 U.S. Open, it was a milestone.
"That was the first major race I had won," Johnson said. "Until that point, I hadn't even won a Grand Prix meet. Just the feeling of winning was nice to have."
Simply put, Johnson's rise to prominence in his sport has been a result of steady improvement.
"I'm stronger now than I was as an undergrad," Johnson said. "I'm much more focused just doing a couple of races. NCAAs and Pac-12s just is way more of a grind. You have way more swims in a short period of time. I would struggle to have to go through that format. I'm better developed for what I'm doing now, physically and mentally."
Making his first World Championship team hasn't changed Johnson much.
"I have my goals that I'm trying to accomplish and I'm very hard on myself about those goals," he said. "Having name recognition or a sponsor or having someone recognize me on the pool deck is not my goal. If I break a world record and one one knows or cares, that's totally fine with me."
What this breakthrough in swimming has done for Johnson is allow him to reaffirm the time invested in the sport.
"The one thing that making these big meets does for me is that it kind of allows me to justify, in my mind, the time that I put into swimming, not that I don't enjoy it," he said. "But, as I progress through (grad) school I have to make a decision how much time to put into this sport. If I'm still really competing at the highest level, I think that's an easy choice for me. Once I'm done with school, that becomes a more serious conversation I have to have with myself."
For now, however, Johnson is enjoying the times of his life.
"As long as I can keep doing it and have fun doing it," he said, "I might as well keep it up."
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