Bulgaria is a lovely country for long distance hikers. Most famous are Rila, Pirin and Rhodope mountain ranges. The Balkan mountains (Stara Planina) are perhaps less spectacularly high, but no less interesting in terms of flora and fauna. The European long distance trail E3, of which the Bulgarian section is known as "Kom-Emine", runs right across the Stara Planina from western border with Serbia to the Black Sea in the east. This magnificent waymarked trail, through old forests of beech and oak, passing remote huts and open grassy ridges with herds of horses running free, will leave a lasting impression on any long distance hiker. A second European long distance trail, E4, leads across the Bulgarian Vitosha mountains south of Sofia, and there are several other well-marked long distance trails and lots of mountain huts in Bulgaria. Bringing camping gear can be handy though.
Inland Bulgaria has a temperate continental climate, with a more maritime, almost mediterranean climate along the Black Sea coast. In the mountains, July, August and September are warm and mostly dry. Nevertheless, be prepared for sudden weather changes and drops in temperature at all times, and fierce thunderstorms in particular.