I am now doing some pushups, situps, toe toushing and have been able to start adding some Nordick Track ski exercise instead of just the treadmill.
Steve, almost all low back pain comes from excessive lumbar flexion (forward bending). When you bend forward the intervertebral discs are pushed backward. Do this often enough and you end up with bulging or herniated discs. Bulging and herniated discs can cause the kind of pain you have experienced.
While it is true that strengthening your "core" can help with low back pain, doing sit-ups is the very last core exercise anyone with back issues should do. Sit-ups are just more forward flexion.
If I were you I would discontinue the sit-ups and the toe touchers and any other forward flexing exercise until you have gone through a proper core stabilization training program like the one here: http://www.vibrantvegan.com/core.html
You need to focus on stabilization and extension(back bending) exercises.
Hope this helps!