Dave Heeley has decided to run one of the toughest marathons in the world, the "Sevens" - 7 marathons on 7 continents in 7 days. The only problem is that Dave is completely blind.Dave Heeley has decided to run one of the toughest marathons in the world, the "Sevens" - 7 marathons on 7 continents in 7 days. The only problem is that Dave is completely blind.Dave Heeley has decided to run one of the toughest marathons in the world, the "Sevens" - 7 marathons on 7 continents in 7 days. The only problem is that Dave is completely blind.