I went to zazenkai yesterday. That means seven zazens and one teisho, with two breaks, a work period (cleaning the zendo), and walking meditation in between, for about five hours of sitting (including the teisho).
For the last ten minutes or so, my legs hurt like fuck. (I'm still not able to sit cross-legged for any serious amount of time, and sitting in seiza or "diamond" position is taxing on both the legs and the back.) When I got home, I went to bed early and slept for eleven hours. In other words, it was tiring, both physically and mentally.
Today, however, I feel very good, also both physically and mentally. I don't think this is a coincidence; there's something to be said for pushing your limits a little bit. I want to do this again, although not right away -- and I think I still have a fair bit of work to do before I can move up to the next step, which would be a weekend retreat.