If you do a single set of half of exercises you need to train each day of the week, rotating these halves, you get 3 and a half sets of each exercise per week.

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.