Criteria for greatest distance athlete:
1) Athlete must have competed in an era in which the entire world was able to compete. No Paavo Nurmi.
2) Must have demonstrated greatness at Marathon. O.P. didn't ask for greatest middle distance athlete. For college and club racing, 5000m is okay. For Olympics & World Championships, there should be no carve out events for almost 1500m runners (5000m) and almost Marathoners (10000m). No one has proven an unique biological characteristic of 5000m &/or 10000m athletes.
Answer: Eliud Kipchoge with honorable mention to Abebe Bikila. What if regarding Samuel Wanjiru.