Just my personal experience so take it as nothing more, OP, but first of all if you are genuinely experiencing severe back pain go to a doctor and not us dumbasses on an internet forum.
I did the standing desk thing for about a month or two and standing 100% of the day is way worse than sitting, imo. It is an impossible to describe feeling of increased stress and exhaustion without equivalent physical exertion that just makes you feel drained with no added benefit.
So I changed my setup so I could transition back and forth from sitting to standing for about 6 months and that was... better, but I still wasn't really feeling any significant benefit.
Then I started taking walks on my breaks. And that made a massive difference. Two 15 minute walks in the day gets your body and blood moving, without needing to be too concerned about sweat and showering like breaking to go to a work out room (although regular exercise is good too, it is just more convenient for me to do that outside work hours)
So my advice is give walks a shot.