I came across the below videos, which is also a good one in addition to what Sudhakar and Mohan mentioned.
Strengthening exercise for Runners
– In this videos they suggest you to do 2 minutes only and you can start doing on the weekdays after your practice run. If you don’t have dumbells or hate weights this one is good.
For sure, marathon runners needs stronger core by doing this if we get six packs I would not say no.
I’d also attached a word doc, which is for athletes on abdominal strengthening. You can do 1 set for each item, which would be good to start with.