I've done a marathon (as in completed and enjoyed, not talking peak performance here) with a longest run of 13 miles, with my physio's blessing - and a caveat that I would need along time to recover after....
What you can get away with as a one off is probably less than you think, but it's not necessarily wise, depending on your longer term goals? For me, it was one that I particularly wanted to do for personal reasons, and I was willing to accept a few months on the bench in return, which wouldn't be an option for you.
Stress fracture sounds nasty though - you might be better asking your physio for advice / finding a physio who specialises in runners rather than forumites who, with the best of intentions, don't really know anything about your injury? I guess the last thing you want to do is get a great time at Nottingham and then have a recurrence of your injury. In terms of building to Comrades, whilst recovering from an injury, would you not be better doing slightly shorter runs but on consecutive days?