Carol Downing is still haunted by memories of last year's marathon. But she's excited about reuniting with other survivors. Michael Dwyer/AP hide caption

itoggle caption Michael Dwyer/AP

Runners train in Ngong, Kenya, in 2012. The country has produced the world's best distance runners for decades, and most belong to the Kalenjin people. Michael Steele/Getty Images hide caption

itoggle caption Michael Steele/Getty Images

Researchers say our brains are probably wired from an evolutionary sense to encourage running and high aerobic activities. Above, a man runs past the Sydney Harbour Bridge on April 22. Ryan Pierse/Getty Images hide caption

itoggle caption Ryan Pierse/Getty Images