Some interesting stats from Brett Larner analyzing who has had the best first half of 2018 for the marathon. The slow Boston Marathon certainly didn't help the US of A.
http://japanrunningnews.blogspot.com/2018/07/breaking-down-marathon-season.html
Women's Top 10 Average by Nation
1. Ethiopia 2:20:23
2. Kenya 2:22:46
3. Japan 2:26:05
4. N. Korea 2:31:07
5. USA 2:31:27
Men's Top 10 Average by Nation
1. Ethiopia 2:04:36
2. Kenya 2:05:05
3. Japan 2:08:38
4. Morocco 2:11:19
5. USA 2:15:35