There's a difference between "CAN" and "SHOULD". Karachi to Lahore or even Islamabad is doable in one-go, with proper rest to man and machine in-between. However, it's another topic whether one should do it or not and it's debatable as individuals' differ in their choices and circumstances.
Had it been me alone or with some friend, I can easily do Lahore without staying overnight. Since I was travelling with family, I kept a leisurely pace and stayed the night at Sadiqabad.