I have a dodgy disc at the base of my spine, which, touch wood, has been OK for the last 18 months.
However, it went on me about a fortnight before FLM 2001: I got it back in shape, and had no pain, after about three days. I then carried on running (I was tapering anyway), and ran the FLM.
My legs were in agony by about 20 miles, and I could barely move by the end. Four days afterwards, I went to see my physio because my legs were still so painful.
And she then explained to me that the problem was my back, and I was just experiencing reflected pain in my legs. She wasn't too impressed when I told her that it had gone a fortnight before the marathon, and that I'd run anyway.
So I would suggest, go see a physio, and see what they have to say.