First off, by "longer rides" I'm saying 20+ miles. I don't ride centuries, or even halfs. A really long ride for me is pretty much anything greater than 30 miles.

I did a 27 mile ride today and about the time I got around 18-20 miles in my back was absolutely screaming. It's all lower back pain and I've noticed that this pain limits my rides far more than energy or endurance. Any pointers as to what I can do to relieve this pain?

For the record, I'm 52, 5'4" and ride either a 49 or 50cm bike.

Quick pic of my basic bike fit.