Kazanlak is the easiest gateway. It sits on the main railway line between Sofia and Burgas — direct trains from Sofia take around four hours, from Plovdiv about two hours via a change in Stara Zagora. The bus is faster: Sofia–Kazanlak takes around three hours by direct coach, Plovdiv–Kazanlak about an hour and a half.
By car, Kazanlak is roughly 200 km east of Sofia via the A1 motorway and an inland road through the Stryama valley. From Plovdiv it is a 90-minute drive across the Sredna Gora. From Stara Zagora, where the closest international flights land (via Plovdiv airport, an hour away), it is a 40-minute drive north over a low pass.
The towns within the valley — Karlovo, Sopot, Shipka — are linked by regular buses along the main east-west road. To explore the rose fields, the side villages, and the higher sights like Buzludzha, you really need a car.