I ask what Maggie does. What are your goals? Racing? Fitness? Just getting out on the road?

I know you don't necessarily want to change the bike, but you could try a recumbent. They really are easier on the back, and some of them are quite fast. If what you want/need is to go screaming down the road, you could do worse than a nice 'bent.

As for your back itself, strengthening the core couldn't hurt, if you start slowly. I think.