The idea behind these long distance runs is to build endurance. Without endurance there can be no marathon or half marathon. A marathon tests your endurance and if you have built up not only your body but also your mind to go the distance you will be successful in the marathon training.o build up your endurance and long run first make sure you are comfortable and able to run 6- 8 miles before attempting a marathon. Once you can do this you should complete one long run every other week, increasing your long run by 2 miles each time or by 10% of your weekly mileage.