I just moved to Istanbul and am looking for hiking buddies or groups, as well as information about trails within reach from Istanbul from people familiar with the area. I found a Facebook group called Hiking in Istanbul which has a hike in October, but would be interested in more frequent outings.
On a different note, I was reading up on the Yenice Forest area in Karabuk and it seems really beautiful. It is also one of the closest hiking spots to Istanbul that I found, other than the Black Sea trails. I am planning a weekend trip there in October, which should make for a very scenic weekend away- foliage, anyone? :) Let me know if you'd like to join.