This has not happened to me (yet), but I have followed up on others   in this situation. I admire your courage and work ethic, but is the   combined impact of many thousands of marathon footstrikes, many of them   under fatigue, the best exercise one could prescribe for your recovery   from disk herniation? The pool running sounds good, but the only other   kind of running I have seen improvement from is with the aid of coil   spring shoes specially designed to minimize impact for injury recovery   (yes, they have been used in marathons). Anything in regular shoes, in   my opinion, represents a risk you must accept or avoid.

Did  the medical personnel try to steer you away from marathoning into   shorter races? Did they indicate whether or not marathoning has   contributed to the spinal condition? My view is that running is good for   the healthy spine, and properly done, tends to make it stronger. I'm  sure the PTs are attempting to insure that your form is optimized to  this end, as well as to sustain this kind of torture,  but the "maybe"  is all about that chance you will lose form and do  further damage.

There is also a fine balance behind strengthening the  core muscles to  better support the spine without increasing muscular  pressure on the  herniated disks. If surgery was avoided, it was probably  because the  herniation did not directly affect the nerve root exiting  the spine. If  there is any chance it will, the marathon is not a good  idea. This  must mean a lot to you, and I believe there could be a  marathon in your  future, but I would spend the month of May evaluating  that possibiity.  If it is a hot race you increase the chance of  fatiguing sooner. The  temperature can rise quickly over a few hours in May.