its about 70 km. Really does depend which side of town you start from. Airport is good because from there you get onto the Boro road v quickly.
Once you get out of Sofia the road is not good. Twisty and not dual carriage way, also it can be flooded in places,plus slushy and icy. Other thing to look out for is the local drivers who will overtake in the most unlikely places! Worst is if there is an accident which can take a long time to clear.