Szlak Karpacki known also as Szlak Graniczny (Border Trail) or Szlak Niebieski (Blue Trail) is third longest marked hiking trail in Poland measuring 430 km. It starts in city of Rzeszów and goes through most remote parts of polish Carpathian Mountains, visiting two National Parks and ending in Grybów. Partially it follow polish-slovakian border and in small distance polish-ukrainian border. It is getting more of attention lately being known as hard due to its remoteness, tough logistics and possible encounters with wild animals (bears, wolves, lynx). Runners will not find any places to rest or resuply often on distances up to 50 km. Trail is marked with blue sign. After completion it is possible to obtain a blue trail badge from PTTK.
Szlak Karpacki can be attempted both ways. It was also attempted and completed in winter.
In 2016 Paweł Pabian finished Szlak Karpacki in 164 hours and 30 minutes in self supported category as described on his blog here https://kronikapodroznika.pl/niebieskie-wyzwanie-2016-odc-11-koncoweczka-i-podsumowanie/#more-2177
Great description of the trail can be found on Agnieszka "Zebra" Dziadek blog https://acrossthewilderness.blogspot.com/2022/06/szlak-karpacki-czesc-2-ustrzyki-gorne.html and her channel https://www.youtube.com/watch?v=NAVbOfNqClk&ab_channel=Agnieszka%27Zebra%27Dziadek
She finished this trail in 2 weeks.