I feel like an old man writing this... only 32 for the record. so I was in tokyo for 2 weeks recently and went to a lot of izakayas and other zashiki-type places. before that I could do a night like that once every 5 days or so no problem... but that 2-week period was probably 10 zashiki evenings, and I can still feel it in my knees and hips when I do yoga now, 2 weeks later.
people that are living there / did for years... did your kahanshin ever get used to it? do you have any tricks for dealing with zashiki?
males would usually sit cross-legged and females do that mermaid thing a lot. Honestly I can do either of those for 6 hours fine... But try that for a week straight and you have problems...no?
There used to be a yoga position I found more comfortable than the regular cross-leg... sorry, can't remember the name right now, but it's like a lower half lotus: the upper feet, instead of resting on the top of the thigh (like in the half lotus), rests in the crevice made by the thigh and the calf (of the opposite leg). It's probably not comfortable for everybody though...
You do eventually get just about as good at it as anyone else in Japan. I lived in a all tatami house for two years (with no real furniture) which helped a lot, and two and a half additional years in Japan. Another thing is that I just have a bad back (and I'm 26!), so I switch to seiza all the time when my back is hurting, even when I'm sitting in my desk chair at my apartmet. I just do seiza on top of the chair.
I can sit seiza for longer than most of my students but not quite as long as most adults. The tea ceremony club does an event at cultural festival where any student or teacher can come do the ceremony, and a lot of my students give up half way but I can make it through a full ceremony with little trouble.