Glenn,
Your writing about your upcoming 8 hour music job brings back memories from the mid-1950’s when I was still in high school. In those days I played accordion in a three piece polka band in the mid-West. We had an 8 hour outdoor gig during the day followed by a 4 hour dance gig from 8 – midnight that evening. We always played standing in those days and no laptops, no MIDI's or no Mp3's. Just a lot of pushing and pulling on the bellows. I know for sure that there’s no way I could ever do that again at my present age. A few years ago I played an outdoor party using my arranger keyboard and that party stretched from the originally planned 4 hours to 6 hours. I was pretty drained by the end of that gig. I barely had the energy to pack up my gear and get home. Age really makes a difference.

Good luck to you. Let us know how things work out.
Paul C.