This fall, 43-year-old long distance runner Dean Karnazes will tackle one marathon a day for 50 consecutive days, running a total 1,310 miles in 50 days. And for each 26.2-mile race, Karnazes and his family of four will travel to a different U.S. state.

Arguably the world's best-known ultramarathoner, Karnazes has already run 350 miles in one stretch, run a marathon in the South Pole, and raced across the California desert in the middle of the summer.

With this fall's challenge, however, Karnazes said on Tuesday he is going a step further in testing the human body's limits.

"I'm curious to see what the limits of human endurance are," he said in an interview. "I still haven't found them."