You can comfortably and quickly get to Borovets by public transport. Every 30 minutes a bus leaves from the bus station "South" in Sofia to the town of Samokov. The trip will take you about an hour. From the central station of Samokov there are buses to Borovets.
Besides, you can book in advance a transfer from Sofia Airport to Borovets at reasonable prices. All popular hotels in the city also provide transfer services.