Alexander Zass (Iron Samson) also trained each day: https://en.wikipedia.org/wiki/Alexander_Zass
"He was taken as a prisoner of war four times, but managed to escape each time. As a prisoner, he pushed and pulled his cell bars as part of strength training, which was cited as an example of the effectiveness of isometrics. At least one of his escapes involved him 'breaking chains and bending bars'."
Rest days are overrated. ;)
Training volume of Bulgarian Method is not much bigger than that of regular training splits like Sheiko or something like that, if bigger at all. What is more frequent is the stimulation of muscles and nervous system paths and BM adapts to that - one does high percentage of one's current max, essentially, one is training with what is available to one's body at the time.
Also, ultra long distance runners regenerate cartilages: https://ryortho.com/2015/12/what-ultra-long-distance-runners...
Our bodies are amazing.