From the thread, I thought Lahore and Islamabad exist on two different corners of globe 
If you're talking about swift 88-90 model, then you'll have to check the condition of the car before leaving. Both motorway and GT road are good. But your swift (if older one with weak engine etc.) may have a tough time climbing on Kallar Kahar (Motorway) or Sohawa (GT road).
I have done Islamabad-Lahore-Islamabad the same day, couple of times on my Nissan. Also went to Lahore on Khyber 91 and came back the next day without any problems.