Like many athletes, Simon Wheatcroft loves chasing that runner’s high, training on a path near his home in the English village of Doncaster. And he never let the fact that he has a rare condition that left him completely blind by age 27 slow him down. Wheatcroft has run marathons in New York and Boston while testing new technologies that, he hopes, will one day help the quarter-billion visually-impaired people around the world to walk, hike and, yes, run without the help of a guide. Lee Cowan reports.