Gwen Jorgensen knows something about transitions.

Six years ago, Jorgensen morphed from a corporate tax accountant who had been a decent swimmer and runner in college into an elite triathlete, a process that culminated in August when she won the first triathlon gold medal for the U.S. at the Rio Olympics.

Fresh off that triumph, Jorgensen is gearing up for another difficult transformation: She will toe the starting line at the New York City Marathon on Sunday to see whether a shift into distance running is possible. No one expects Jorgensen to win her debut marathon, but her attempt to turn the usual evolutionary pattern of triathletes 180 degrees has set the endurance world abuzz with the possibility that she may be able to excel in two vastly different events.

“She will be very good, probably the best triathlete ever to run marathons,” said Bobby McGee, a performance adviser at USA Triathlon, who has advised both Jorgensen and the 1996 Olympic marathon champion Josia Thugwane.

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For her part, Jorgensen, a 30-year-old Wisconsin native, isn’t making any promises, in part because she seems predisposed to avoid boastful predictions. She is quick to point out that she wasn’t an outstanding runner in college, and that the marathon will be about 60% farther than her longest training run. Since Rio she has tried to incorporate the kinds of workouts that elite marathoners do regularly while she continues to train for and compete in triathlons. Whatever happens, she isn’t expecting to feel very good during the final 6 miles of Sunday’s race.

“I think I have the engine but my muscles are unprepared,” Jorgensen said in an interview last week. “For me this is a learn, search and discover mission.”

When her run ended, Jorgensen was the first American to win the triathlon gold medal. Photo: Liu Bin/Zuma Press

On the surface, Jorgensen’s transition wouldn’t figure to be all that difficult. She takes a little less than two hours to complete the Olympic-distance triathlon—a 0.93-mile swim, a 25-mile bicycle ride and a 6.2-mile run. Elite female marathoners complete the 26.2-mile race in less than 2½ hours. Jorgensen is capable of running the final 6.2-mile leg of her triathlon at a 5-minute, 30-second pace (and that is after 80 minutes of intense swimming and running), which would put her under 2:30 in the marathon if she can keep it up.

Now the bad news. Coaches and experts in sports science say high-intensity running is a lot harder on the body and wears down muscles far more than swimming and cycling.

“In the Tour de France, cyclists race at such a high level for three weeks,” said Mark Allen, a six-time winner of the Ironman World Championship. “There is no way you can run at that same level for that long.”

In a 2013 study, scientists at Appalachian State University’s Human Performance Laboratory subjected 13 long-distance runners and 22 trained cyclists to 2½ hours of either running or cycling at 70% of their maximum effort for three consecutive days. That represented a significant increase over their normal training volumes. Blood tests for markers of muscle damage and inflammation were 133% to 404% higher in the runners. The runners’ inflammation markers were ​as much as ​256% ​more prevalent. In addition, the runners described their level of delayed onset muscle soreness as 87% more severe than the cyclists.

The results confirmed long-held beliefs that the motion of running is more taxing because it requires what is known as “eccentric contraction.” That means the muscles have to exert force when they are lengthening rather than shortening, which creates more damage and micro-tears.

To be fair, Ironman Triathlons, which tack a marathon on the back of 2.4-mile swim and a 112-mile bicycle ride, are likely more taxing on the average body. However, plenty of competitors who done both say they recover more quickly from the slow burn of an Ironman, in which the marathon is run at a slower pace, than a standalone 26.2 mile race.

So while Jorgensen’s body is plenty taxed after 80 minutes of swimming and cycling, the wear and tear she will experience Sunday at the 80-minute mark promises to be much worse. She will likely be heading​onto the Queensboro Bridge toward Manhattan at that point.

Jorgensen says running is her favorite of the triathlon disciplines, and she has always wanted to run the marathon. In college she mostly competed at 3,000 and 5,000 meters, and couldn’t keep up with the ​top ​specialists at those distances. To prepare for the marathon, Jorgensen has completed a 16-mile run and has done some long-distance speed training. An occasional workout might include three intense 3.1-mile intervals or three 4-mile runs.

Gwen Jorgensen of United States in action during the ITU World Triathlon on June 12 in Leeds, England. Photo: Tom Dulat/Getty Images

What at first seemed like a quixotic quest got more serious last month when she placed third in the U.S. 10-mile national championship race in Minnesota, finishing in 53:13, a 5:20 pace.

McGee, the USA triathlon high-performance specialist, said Jorgensen is embarking on this experiment with a lot of genetic advantages. The two most important numbers for a long distance runner are the “VLT,” or velocity lactic threshold, which measures how quickly blood-lactate levels rise when someone is running at a marathon race pace, and the “vVO2 Max,” or velocity VO2 Max, which is her speed when her heart and lungs are running at maximum efficiency. For Jorgensen, both are in line with those of world-class distance runners, according to McGee.

In addition, at 5-foot-10, Jorgensen’s roughly 1.7-meter stride length is longer than that of many of the world’s elite marathoners, who tend to have compact bodies. That allows her to complete the race using fewer steps. Also, her stride rate, which is about 186 strides per minute, isn’t substantially lower than the stride rates of top distance runners. And while her size would be a disadvantage in hot weather, because smaller bodies with less surface area stay cooler than those with more surface area, it shouldn’t been an issue during fall and early-spring marathons.

McGee called Jorgensen “an outlier,” but Jorgensen, ever the pillar of Midwestern humility, has little use for the happy talk. “I go into this race with no expectations and I will be unprepared,” she said.

No one is expecting a miracle, but in the often predictable world of endurance sports, where athletes tend to stick to their best events, the experiment itself is stirring a curious sort of excitement.

“She’ll be really impressive,” Allen said, “She ultimately may not be good enough to win a big marathon, but that is sort of beside the point, isn’t it?”

Write to Matthew Futterman at matthew.futterman@wsj.com