transport links to Borovets are exellent. In act it is probably the best connected ski resort in BG.
It is approx one hours from Sofia airport by car. You can take the scenic route past Lake Iskar (good for watersports) or you can take the morotway via Kostenets.
Either option is about an hour but if you get stuck behind traffic on the scenic route then it might take a little longer.
The roads between the various towns/villages around Borovets are also in resonable condition by Bulgarian standards.
Borovets is about 1.5 hours from Plovdiv which is the second biggets city in BG and also has an airport.