As one with both back and leg issues, I try to be careful and build muscle while avoiding strain. It's way too easy for me to favor a weak area and end up creating other problems. One simple exercise that has really helped is to stand or walk with my hands clasped behind my back. It seems so basic, but it really helps stretch and build your core muscles, and those really lose strength when you're injured.
I have to be especially careful when lifting, since my knee and back issues make it difficult to lift with my legs. Some days even lifting a full hay tub causes my back to spasm. I try to stretch my back before any lifting, which really seems to help.
I injured my back when I was in my 20s, and this haunts me today. I have to avoid cantilevered stress -- I hold everything, even light objects, close to my body and/or create a bridge.
We recently covered our chicken run with netting, which means that to get to the other end, I have to bend over and duck my head, but at least the heaviest things I carry in there are small pitchers of water and feed, and this is actually turning out to be good exercise.
There are days that my back hurts so bad that I give in and let Keith feed everyone, but I figure it's "use it or lose it," and I have no intentions of losing it!