Mo Farah's Marathon Debut Ends Midway Olympic champion Mo Farah stuck to his plan to run just half the London Marathon in preparation for his debut next year at the full 26 miles. The Briton was among the leading group halfway through the race Sunday but slowed down and stepped off the course near Tower Bridge. Farah is trying to get accustomed to the course and has "learned a lot." He says his biggest challenge is "picking up the drinks and making sure you pick up the right drink, and I made a mess of it." Farah won the gold medal at 5,000 and 10,000 meters at the London Olympics.