English athlete Roger Bannister among a crowd at Oxford after becoming the first person in the world to run a mile in under 4 minutes (3:59.4). Norman Potter/Getty Images hide caption

itoggle caption Norman Potter/Getty Images

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