I found an article on Marathon Etiquette on http://www.lifemojo.com/lifestyle/race-etiquette-for-runners-3356989. Having read the article, in my opinion Marathon etiquette basically comes down to this:
3. Easy on the groaning, grunting and wheezing; not nice to listen to for your fellow runners! We are all in pain, just no need to make the other runners feel miserable;
4. Don't stop suddenly to walk on the course or at water stations; move to the side of the road! Ofcourse, when in a mountain marathon, it's perfectly fine to walk on the course and to hang around at water stations;
5. Don't suddenly change directions or cut off other runners; this one is a no brainer, actually;
6. Smile at and say 'thank you' to the race volunteers; OK, smiling might be a bit much to ask for at some point, but do say 'thank you' when handed a cup of water.