WASHINGTON — Two years ago, Amanda Sullivan’s friends, a group of severely wounded military veterans who she connected with through Facebook, challenged her to join a gym. Using a wheelchair at the time, Sullivan, 36, scoffed at the request.

“I’ll be the only person who’s disabled at the gym in a wheelchair. What am I going to do in a wheelchair at the gym?” she remembered saying. “And they said, ‘you’ll never know unless you go and try.’ ”

She did — and she’s never stopped. On Sunday, Sullivan, a civilian who was injured in two automobile accidents, crutched her way through all 26.2 miles of the Marine Corps Marathon in Washington, D.C., finishing the race with an infectious energy and wide smile.

Donning Superman-themed knee-highs, she competed to honor her boyfriend, Cpl. Todd Love, a medically retired Marine who lost both legs and an arm in October 2010 after stepping on an improvised explosive device in Afghanistan. He was one of the guys at Walter Reed National Military Medical Center who pushed her to take those first steps when she thought she never could.

Sullivan met Love and the others after spending about three years on bed rest, the result of two car accidents in New Jersey that left her with traumatic brain injuries, nerve damage and herniated disks.

In the first accident in early 2009, Sullivan said she was about to enter a parking lot when her car was hit by another driver. Less than two months later, she was walking into a physical therapy center when an elderly man rammed into her. She flipped on to the hood of his car and then careened onto the pavement.

During her recovery, Sullivan shuffled between her childhood home in Short Hills, New Jersey, and New York City, where her father lives. It was as emotionally disastrous as it was physically taxing for Sullivan, a former high school varsity field hockey, lacrosse and basketball team captain who relishes in helping others.

“I lost the outlet for me, which was always sports and doing so many different things,” she said.

Sullivan had once moved to Mexico where she ran an orphanage and taught Mayan, after graduating from Villanova University in Philadelphia. Later, she was a first responder for Hurricane Katrina in New Orleans, and worked as an aid worker in conflict and disaster zones in Jamaica and South America. She had moved back to the U.S. to open an orphanage — a plan that was derailed after her first accident.

But thinking about the impoverished children she had met during her travels would finally snap Sullivan out of a depression. Following months of suicidal thoughts over her lack of mobility after doctors told her she would never walk, she stopped feeling sorry for herself and decided to embrace her disability.

“If I learned anything from sports, I learned that it doesn’t matter what the scoreboard reads at halftime,” she said. “It could say Sullivan zero, life 100. But if there’s still time left in the game, that means I can turn things around.”

And she did, with the help of her friends at Walter Reed. With their encouragement, she began experimenting with forearm crutches, and gradually transitioned out of the wheelchair. She began slowly, using the hand cycling machine at the gym and then taking steps on the treadmill. Even though she could barely feel her right leg, she could use the treadmill to pick herself up with her arms and swing her body in a walking motion.

In 2012, two of her friends from Walter Reed challenged her to participate in the Tunnels to Towers 5K race in New York City. Sullivan was hesitant, but once she realized Todd Love was entered in the race, she agreed.

“I had a crush on Todd so I thought, well I might die during this race, but at least I’ll get to hang out with Todd before I die,” she said, laughing.

The couple spent the days leading up to the race together and had an instant connection. They started dating soon after the event during which Love competed in his wheelchair and Sullivan on crutches. They’ve been together ever since.

When it comes to staying active, Sullivan has met her match. In the past two years, she and the 24-year-old Love, who live together in the Atlanta area, have participated in about six Spartan races and road races together. Love also surfs and skis. And with his encouragement, Sullivan fulfilled another goal: skydiving.

“I guess we’re both kind of on the same level of crazy,” she joked. “Being with Todd is amazing because I see how he adapts and overcomes all of the different things that are in his path, and it makes me realize that I can do the same thing.”

For Sullivan, competing in the marathon was one of them.

Completing 26.2 miles on crutches wasn’t easy. At one point she said her whole body shut down, but she pushed through and finished the course in just over seven hours. Love was waiting for her at the finish line.

“I was a little worried at first, but the more I thought about it, I was like, she’s got it,” Love said after the race. “I knew she would finish.”

It was worth it to Sullivan. She hopes that others with disabilities will look to her example. She is the New York metro ambassador for the Abilities Expo, an event for people with disabilities and their caretakers. She wants others to realize that their disabilities need not limit their potential. Sullivan is hopeful that one day she’ll be able to walk without crutches.

Walking the length of the course for Love, as well as the Marine platoon that helped save his life, and all of the veterans who can’t take their own steps, made the marathon experience that much sweeter.

“It was just an honor to be out there and to honor not only Todd, but all of his brothers who kept him alive that day,” she said.