The starting point of my run was Sinemorets and the end point was Silistar Beach. The distance between the two is about 7 km, mainly a good road. However, the road is narrow and I don't recommend running there after 10-11 a.m. due to high traffic. It's perfect for morning runs, though.
Silistar Beach is one of the most beautiful beaches of the Bulgarian Black Sea. It offers clear sea, wild forests and beautiful rocks. It's located far to the south and is only a few km from the southern border of Bulgaria near Rezovo. Temperatures there are usually a little higher, but due to the distance from the major resorts (like Burgas, Varna, Sunny Beach), the region is less crowded.
The reason I got attacked by flies is clear - sweating. It's happened to me before, but not on such a large scale. The feeling is really disgusting because you feel bugs constantly hitting your face literally every second {and sometimes they go in your mouth). One way to escape is to run faster but they are always coming after you so no escape, haha.
Special thanks to the guys who gave me the garbage bag. Go visit their place next summer and enjoy Silistar beach and Bulgaria!