Lower back pain cause? How much of lower bike pain over longer rides due to bike fit or body positioning versus core strength?  I have spent many years "fixing" my fit but have never had a professional fitting.  I have also ebbed and flowed on my core strength over years and can't pin down it's role in "saving" my lower back pain over time.
I often experienced lower back pain during and after rides on my roadie. Stretching helped me significantly. Try regularly stretching the muscles below, above and around your hip / bum, perhaps using hip-focussed yoga poses.