Sam123 said:
There is no bike fitter around my area.
Can you still get fitter with back pain?
There's no LBS near you or within driving distance? It's worth driving 2 or 3 hours to get a proper fit, especially when you're hurting on the bike.
Can you get fitter? I guess depends on what you mean. Back pain will limit how hard you can pedal and limit you to low intensity rides. That could be good for burning fat, but the onset of your pain is not long after you start riding, 20-25 minutes.
Frankly, I think what is more likely is that without some sort of intervention (proper fitting, at least), you'll find that the back pain worsens and maybe starts occurring earlier in the ride. If continued, a back injury likely awaits.
One thing that helps a back on a bike is to have good core strength. If you can do exercises targeted at your core, and do them without pain, that will put you in better stead for riding.
I went through a spate (1+ year) of neck and back pain, and it caused big changes on the bike, namely I raised my stem quite a bit during the period of pain. The point of this is to say my rides were shorter.....and fewer.......and not very enjoyable. I had to change my priorities and put my back and neck first, meaning that sometimes I just couldn't ride (up to weeks). You don't want that.