So I've been playing since I was a kid, but as I got to my 40s I played less because after 20min (or sometimes as little as 5m) my back hurt. Went to physical therapist and back doctor, and both said where my pain was wasn't an area they would expect anything serious and suggested back exercises that didn't help. Finally a few years ago I googled "back pain for pianists" and discovered the Alexander Method. This is a complete set of information regarding posture (back, legs, arms, fingers, head) targeted at people who play 6 hours/day. Overkill for me, but I did read through a book on it ("What Every Pianist Needs to Know About the Body Thom Miles, Thomas Mark, Roberta Gary") and arrange an hour session with a piano teacher versed in the method. We identified my issues, and now I can do 2 hour gigs without issue. Just passing it forward in case anyone else is having the same problem.
best
/j
best
/j