Good luck on your century! I did my first one last year and loved it.

I also get back pains. Strengthening your abdominal muscles definitely helps support your back. Crunches and leg lifts really help.

But even after stretching and strengthening and checking the bike fit, about 60 miles into the ride, I still get pains. I discovered that 400 - 600 mg of ibuprofen about 1/2 an hour before the ride stopped me from hurting. I would do all of that other stuff first, but if you still hurt, you might try this.