Thanks Neil & Six Physio for advice.
I ran 13.5 m on Sunday and my lower back started aching very badly and I was in a lot of pain throughout the night. Ibuprofen did the trick, and I'm now ok, but I think it gave me the wake-up call I needed - there's no way I'll manage a marathon, pounding tarmac for over 4 hours. (Annoyingly I ran 14m the previous week and was ok),
Physio has booked me on a 6 week back care course, which will focus heavily on core work, relaxation, and looking at my "slings" so that I'm given appropriate exercises. She said she'll then get me doing lots of core and transverse abdominal strengthening. I'm to continue with McKensie stuff for the actual disc bulge. She said to try and stick to treadmill running and to stop going to running club at the moment and just do stuff by myself. Worse case scenario, if I'm still getting nerve root pain, she'll refer me to a consultant. I guess discsectomy would be the last resort.