(CNN) -- The California runner who was lost in a forest for three days without water had barely a few hours to go before she would have died, her doctor said Thursday.



Runner Maria "Gina" Natero-Armenta, 36, survived for three days without water in a California forest.



Maria "Gina" Natero-Armento, 36, not only survived 72 hours with only a slice of apple in her stomach and a little bit of water for nourishment, she also has only one functioning kidney, Dr. Derrick Hong said.



He spoke with CNN on Thursday afternoon, along with Natero-Armento and her husband, Armando Armento, in a conference call interview from her room at Mission Hospital in Mission Viejo, California.



Natero-Armento is no amateur when it comes to distance or mountain running. She's one of the top female finishers in a San Diego 100-mile race and an experienced ultrarunner with top times in other 100-mile and 50-mile races. Her body was unusually strong to begin with, but she also has one kidney -- the other doesn't function because of a congenital disorder.



"This is extraordinary," Hung said.



She said she had planned a simple eight-mile run, a small fraction of what the ultrarunner usually tackles in a sport that challenges competitors to run at least farther than a 26.2-mile marathon. She is among the majority of ultrarunners who like doing 100-mile races.



She set out from her Oceanside home at 5:30 a.m. Sunday to meet Fidel Diaz, her running partner and brother-in-law, who is also a serious ultrarunner. They planned to run along a trail in the the Cleveland National Forest near San Diego.



She brought along two water bottles and wore a Camelpak, a backpack that can be filled with water. That was plenty of water for an eight-mile outing. She didn't bring food with her, but Natero-Armento said she ate a piece of apple before she started, and that was the only food in her stomach.



Natero-Armento said she did several things that were out of character that day, and she wishes she had been more prepared.



"I am very careful usually, but that particular day, I was not," she said. "I normally, the night before the long runs, I have everything ready. And this night, I had nothing ready, and it was just a mistake."



She usually wears a Garmin GPS watch, a sophisticated device that runners use to find out where they are, the distance they have traveled, calories burned and altitude.



"I didn't have my Garmin and wasn't wearing a watch," she said. "I don't know. I always carry food with me, and I didn't have nearly enough."



"I always carry my phone with me, and I didn't have my phone with me, and that really was a big mistake," she said. Feeling antsy to just get on the trail and run, she wasn't thinking deliberately. "I just wanted to get going that day; get some fresh air and go for a run. I hurried up."



Natero-Armento and Diaz began running about 6:30 a.m. Sunday, she said. She lost track of as much as eight hours, she said, as she and Diaz became lost. By then, she was dehydrated and disoriented.



And Natero-Armento said she also made another, more serious bad decision. At some point in the run, Diaz had become ill. Ultrarunning is a sport in which some participants sometimes push themselves way too hard.



"[He was] pretty much beside me or in front of me, but I do know that I was insisting on keeping going since I was OK," she said. "He doesn't eat or drink much ever [when out on runs], so I know he can handle that."



She said that she kept going because she'd seen him run hard while sick -- also not uncommon in 50- and 100-mile races -- and that she figured
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