FlyerTalk Evangelist
Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,186
Yes, perfectly doable and pretty easy since the metro has opened.
You could take the metro lines U1 ( M11 ) + M2 from the airport to Haliç Golden Horn in about 45-50 minutes one-way and have time in the Historical Peninsula.
Most of the sights would close at 5pm in the winter and 6pm in the summer, so you would need to act fast if you would like to go into them such as the Topkapı Palace, etc...