I definitely think Hall is a medal contender.
History has shown that the Kenyans and Ethiopians don't bring their A games when it comes to the olympic marathon. Geb hasn't had success in running competetive marathons. He aces the time trial affairs but look at London the past couple years. He essentially walked it in two years ago and dropped out last year. Last year he complained of allergies which affected his breathing so I wonder how he will deal with the terrible air quality he is going to be faced with over in China.