In reply to loose overhang:
Sorry my last post could have been more helpful.
When I was in Bulgaria, a tour guide mentioned a long distance trail that traversed the entire, I assume Balkan mountain range, so what you want to do is a thing. A little googling suggests it is called the “Kom-Emime” and is also part of the European Route E3.
The Bulgarian part of E3 route is 720 km long and follows the main ridge of Balkan Mountains from Mount Kom at the border with Serbia to the Black Sea coast – Cape Emine. It is actually the first and most popular long distance walking route in Bulgaria, simply known as “Kom-Emine”. Crossed for a first time in 1933, it become extremely popular during communist times, when large organized groups used to cross this route in summer. In this period it was known as part of the “Fellowship Route” between the countries from the ex-communist block. Most people manage to cross it for a period of 20-25 days.
https://bulguides.com/european-long-distance-walking-routes-in-bulgaria/
You could do worse than PM zv and ask them to contact their friends for more information.
I would highly recommend a visit to Museum of Socialist Art in Sofia and there is a walking tour of Sofia focusing on the Communist times. Doing this may give you a little understanding of the background of this fascinating country. Though it is in Europe, its seems very different to Western Europe. I suspect the walk could be very interesting, Bulgaria is undergoing one of the steepest drops in population if not the World, certainly Europe, so is a changing country and traveling through the rural areas and smaller towns may be very interesting.
Hmm, I could be talking myself into returning to Bulgaria.