Eurasia Marathon is the one you are asking for. It is organized every 3rd Sunday of Octobers. It has both half and full marathon format if I'm not wrong. I did the 8km fun run once crossing the Bosphorus bridge. It was amazing. For more info:
During the Avrasya Maratonu in November you will be crossing the bridge and run from Asia to Europe.
The Buyukada Run is organised by New Balance and takes place on the Prince's Islands.
...and check out more about running in Istanbul here.