Bekele and Gebreilasse both ran paced all-out 5 and 10Ks and their rates of decrease in pace were identical. The rates of decrease in pace for the 2 continued all the way through the marathon. Runners typically slow down by 25-30 seconds per mile from 10K to the marathon. If someone can decrease their pace from 5 to 10 at half the normal rate they can do the same thing from the 10K to the marathon, but nobody has ever managed 15 seconds or less.
In regard to Rupp, he ran 13:25/13:19 for his 26:44 and considering his 1:57 finish it was worth 13:15 or faster for the second half. Do people really believe that if he had run a 5K that night with pacers he would have run 12:59?