I had a lot of trouble with a tight lower back and back pain until recently. It got so bad it hurt when sitting and standing for more than 5 minutes. Couldn’t ride without pain for more than an hour.
Someone on here recommended the guys from ‘Upright Health’ on youtube (to someone else, but it was a great recommendation, I find their videos excellent!), and I just tried different excercises they had, until I found some I was really (and surprisingly) bad at, and then just worked on that. I’m much better now, no more pain when sitting or riding, and as a side plus my tight calves when running have also gone away (better hip movement I think).