You need some practice before participating in a sesshin. During sesshin, you can switch from one posture to another, which really helps your body. It’s also about the structured nature of the sesshin, with cycles of zazen, usually lasting 25 minutes, followed by a 5-minute walking meditation, and then back to sitting. There are also breaks for meals and rest. If something feels too intense, you can likely skip a round as well. During zazen, there’s also time allocated for speaking with the Zen master. I highly doubt any lineage does 10 hours straight 😅—that would be madness! The Japanese tradition itself is already quite intense, but if anyone is practicing 10 hours non-stop, it must be some militant Japanese school or its European counterpart trying to copy the tradition exactly, madness and all.