It is a tough call! I would not suggest biking on the streets with a lot of traffic. The cars usually change lains pretty rapidly, and some people don't follow the regulations. The main reason for this is stress. Most people spend at least 2-3 hours commuting/driving daily.
If you would like to have a pleasent bike ride, then I would suggest either going to one of the princes' islands for a day trip and renting bikes there. You can also ride your bike along the coastlines of Istanbul. Along Bosphorus, along Marmara sea would work. Belgrad Forest would also be a great option. You will see a ton of intense bikers there.
