The marathon is a unique challenge. In the marathon you receive rewards for completing all the daily quests every day. The more sets of daily quests you complete, the better the reward.
The best marathon prizes are awarded for completing 7 days worth of daily quests. But if you didn’t manage to complete all the dailies on one of the days, there’s no penalty. You'll receive 3 new dailies the next day and can continue working on your marathon progress.