(CNN) Cindy Martinez joined the Marines when she was 17 years old.

"When you go through Marine Corps boot camp or any type of training in the military, you're trained to be prepared for something big," she said.

What Martinez didn't realize was how much she would need to rely on her mental toughness, not on the battlefield but in civilian life.

Martinez served four years and met her husband, David, while on active duty. In May 2015, she was working full-time and raising two small children when her strength was put to the test.

Cindy Martinez with her husband, David, and two kids, David and Bianca, before she got sick.

"One day, I woke up, and it just felt like a little ache on my left side, in my back shoulderblade area," she said.

Martinez thought she must have slept on it awkwardly. But by the weekend, she was in intense pain.

"David took me to the emergency room, and from there, I don't remember much," she said.

Fight of her life

Martinez was diagnosed with necrotizing fasciitis, a type of flesh-eating bacteria. She doesn't know how she contracted, it because she didn't have any visible cuts or wounds.

Doctors removed the bacteria from her back, but her body was in septic shock.

"Her life was now in danger," said her husband, David Martinez. "I tried to be as strong as possible for her and for the kids."

The medications that saved her life also ravaged her arms and legs.

"My vessels were constricted, which didn't allow blood flow to my extremities because the doctors wanted all the blood to go to my heart," she said. "They told my husband, it was life over limb."

Martinez fought for her life after being diagnosed with flesh-eating bacteria.

Martinez underwent multiple amputations: both legs below the knee, her right arm above the elbow and portions of all the fingers on her left hand.

"When I woke up and the amputations were done, it's a hard thing," she said. "They're gone, and they're not coming back."

Martinez spent almost three months in the hospital and seven weeks in rehab. She learned to walk again with prosthetics.

But coming home wasn't easy.

"It's almost like you're a different person," she said. "When your husband is at work, your kids are at school and you're home by yourself, that can take a toll on you."

Finding new strength

Martinez found support in an unlikely place. She reached out to CrossFit GOAT (Greatest Of All Time) gym in Dacula, Georgia, to see about becoming a member.

"I still needed to work on myself and build my endurance to walk on my prosthetics," she said. "So I thought, 'Let me see if CrossFit could be something I could do.' "

Amanda Greaver, founder of the gym, had never coached someone with multiple amputations. But she was up for the challenge. "It was a lot of trial and error, but Cindy has such a positive spirit," she said. "She's always up for anything I throw at her."

When Martinez arrived at the gym, she could barely lift 5 pounds. But with modifications and adaptive devices, she can now squat 72 pounds and dead-lift up to 95 pounds.

"Working out has really changed my outlook on life. It's my form of therapy," she said. "Now I can pick up my daughter, and I can pick up my son if I want to and hug them. ... I just can't put it into words."

After regaining her strength, Martinez sought out a new challenge. She signed up for the Marine Corps Marathon in Washington in October.

Join the conversation See the latest news and share your comments with CNN Health on Facebook and Twitter.

"I did the first 25 miles on my (modified) bike, and then I changed into my running prosthetics for the last 1.2 miles. My husband was there to finish it off with me," she said.

"I was extremely proud," her husband said. "I could just see that determination on her face. ... No matter how tired she was, she just couldn't stop."

Martinez completed the Marine Corps Marathon in October.

Martinez was overcome with emotion when she crossed the finish line.

"Just a year prior, I was in a hospital bed," she said. "Whatever challenge (you face), there is a positive in everything. ... You just have to be willing to try something new and put yourself out there."