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Marathon runners get a huge amount of respect - and rightly so.
Running those 26.2 miles is a gruelling feat that takes months of training, often in all weathers.
But American ultramarathon runner Dean Karnazes has run a whopping 350 miles without stopping.
His 2005 journey went from San Francisco to the California wine country before finishing at a running track at Stanford University.
He told Runner's World in a live update: "After 300 miles I started stumbling and weaving in traffic. For my safety we measured the distance to the Stanford University track and I'm headed there to run the last 590 miles."
He made to beyond-epic journey to raise money for a one-year-old girl named Katalina who needed a heart transplant.
He told A Trail Runner's Blog: "During the times when I wanted to stop, I kept reminding myself that my pain was nowhere close to the pain Katalina and her family are enduring."
Dean's amazing achievements are partly due to genetics. Watch the video above to find out more.