Geb's performance in Berlin, coming after his 10k focus this summer, can be a good reminder to many marathoners the importantance of touching base with our shorter distance speed on occassion.
It is a good idea for marathoners to interject short cycles of shorter distance training and racing (5k-10k) into their overall schedules on a periodic basis. Not necessarily between each marathon but doing so once every year or so is probably not a bad idea.
Even if you aren't setting PR's at the shorter distance the shorter focus help to improve your aerobic power and gets the body use handling a faster pace. This can translate into marathon pace seeming easier and improved paces being able to be handled during the marathon training cycle itself.
For many marathoners a yearly training pattern may look like this.
20 week cycle focused on a spring marathon
12 week cycle focused on 5k/10 during the summer
20 week cycle focused on a late fall marathon