London Marathon: Mo Farah faces tough challenge - Mike Costello

BBC Sport athletics commentator Mike Costello tells BBC London that this year's London Marathon will be "one of the toughest challenges" of Mo Farah's career.

British double Olympic and world champion Farah ran half of the race in 2013 and will make his full debut in the event on 13 April.

The 30-year-old will face world-record holder Wilson Kipsang and Olympic and world marathon champion Stephen Kiprotich over the 26.2-mile course.

Costello says the race is "unknown territory" for Farah, adding: "If he wins, it will be an achievement to match anything he has done in his career so far."