Walking is an easy way to strengthen your back muscles. Another good way is pilates. You can pick up a basic tape or dvd like Pilates for Dummies that can be very helpful. It is a really good idea to try and strengthen your back. You can avoid alot of problems that some of us are dealing with. You may want to ask your doctor about your sit-ups. They may be more harm than good if you do not do them right.
There is a great one where you suck in your belly while lying on the floor, tighten your butt, and try to make your belly button meet the floor. This makes your pelvis tilt up. You are trying to suck everything up through your belly button. I was told to breath out during this, which makes it like trying to rub your head and walk at the same time.
The sit ups that we see in school are usually done incorrectly. There is a very specific way to do them that doesn't hurt your back. I lay flat, raise my knees, then put my hands behind my neck but not grasping it. They are simply resting there. I raise up, looking straight up, imagining an orange between my neck and chest. I only raise a few inches and hold it. At the same time, I am sucking in my belly while breathing out. This tightens up the abs/core muscles.
You know that you are making progress when your abs start feeling it.