WASHINGTON -- Nat Shaffir runs six miles a day, six days a week. He started running when he met a college student who had just run a marathon.

"I looked at her and said, she did a marathon? If she can do it I can do it," he said.

He was 65 years old at the time. Now he's 81, and later this year hopes to run his 12th marathon. When he runs, he doesn't listen to music or plan his day. He says his mind is usually in a much darker place.

Shaffir gives tours at the Holocaust Memorial Museum in Washington, D.C., where he shares his own story. He was born in Romania in 1936 and had a happy childhood until he was almost 6 years old, when everything suddenly changed.

Nat Shaffir and his family United States Holocaust Memorial Museum

"One of our neighbors was a priest," he explained. "He showed up with a police officer and two armed guards, and he's pointing at us and he's saying... 'these are Jews.' So we were actually turned in to the authorities because we were Jews, by a priest."

His family spent years in a Jewish ghetto, with barely enough food to survive, and 32 of his relatives perished in Nazi concentration camps.

Shaffir, though, eventually made it to Israel, and at age 24 to America. Running, he says, gives him strength.

As a young paratrooper in the Israeli army, he was shot in the knee. We asked him if he has any bullet fragments in his knee. But he said he's actually had a knee replacement. Yet he still runs marathons. But running, while re-living the Holocaust, also gives him the mental strength to keep telling his story.

Nat Shaffir still runs marathons Nat Shaffir

"Running gives me so much more incentive to speak out and make sure things like this don't happen again," he said.

He's 81 years old -- and living stronger than ever before.