RIO DE JANEIRO — The Women’s Marathon will kick off at the Sambódromo at 8:30 a.m. EST Sunday. Mare Dibaba of Ethiopia, the favorite, won the world championship marathon in Beijing last August. Look for a challenge, though, from Kenyans Jemima Sumgong and Helah Kiprop, who finished just one second behind Dibaba in that race. And while no American woman has won a medal since 2004, Shalane Flanagan, running in her last Olympics, may have a shot. She grabbed tenth place in London in 2012.