I travel stansted to Dalaman via Istanbul several times a year and in my experience you need at least and hour and a half between flights, even if they are on time, as Istanbul is always busy.
We missed our connection couple of weeks ago and Pegasus just booked on onto the next flight two hours later. I guess this would depend how full the flight was.
Never used fast track but their queues are often very long too. It's difficult to book from uk as when I recently tried it did not accept uk cards. You can buy at airport but if your short of time it's impossible.