Long Run Post-Operation

It's 12 days before the Manchester marathon!!  Obviously due to the head operation at the end of February, my running in March has been kinda limited!!  And my running for the second half of February was pretty much non-existent too!!  Not great training for the marathon, but if I am going to give it a go, what do I need to do?  Clearly the aim now is not to worry about pace, but just try and get round it, ideally without stopping!  Even breaking four hours is not a goal - that's too quick for me in my current state.  But maybe 6m/km is achievable for a marathon.  That would equate to about 4:13h which is, if anything, faster than I should be aiming at!!  The not stopping is bizarrely important to me, because I have a possibly daft feeling that the knee/calf problems I have had in long runs only happen after I stop.  If I keep going, even very slowly, the pain won't start until I stop, which is hopefully after the finish!  So I am looking for a pace that I can maintain for the full marathon without stopping.  6m/km perhaps, or maybe even a bit slower - not sure!  But to get to a position where I can keeping going, I really need to run 20 miles before the marathon.  And that should be at least a week or two before the marathon - not simply a few days!!  I would want to build up over February and March to 20 miles, with maybe weekly long runs of 14, 16 and 18 miles or something like that.  But my hospital visit means that's not possible, and I still want to do one 20-miler, and the weather was good today, so today was the day!  I was prepared today - I even took energy gels!!  And I ran slowly (my average pace for the run today was 5:57m/km), so it was all to plan!!  But 20 miles is about 32k, and I managed 30k and had to stop!!  The first 23k was ok and I felt fine, but the last 7k were just very, very slow because I was feeling done in!!  The pace for the last 7k was over 6:30m/km, which is essentially the average pace I did last year when I was walking for about 15km of the run!!  But I was struggling today!  I can do a half marathon ok, but beyond that it just gets too hard!  Today my half-marathon time was 2:01h which is slow but roughly 4-hour marathon pace.  That pace is very comfortable for a half, but I can't do it twice in a row!!  I don't have time to get confident that I can do a full marathon at any pace.  Today, even though the last few kilometres were so slow, I couldn't keep going even at that slow pace for another 2k, never mind another 12k!!  I honestly thought that I could keep running slowly forever, but I can't!!  If I can only do 30k, and even that is a huge struggle, I have no chance of doing 42k!  And I might fall (possibly very bad for the head!!) or get knee/calve problems like before.  Medical people I know have said for weeks that I should wait until next year, and not do the marathon this year.  Today's run has made me feel that they are absolutely right!!  I just don't think I can do it!  My current thinking is I need to pull out of the Manchester Marathon this year, but I'll sleep on it before deciding.  And I am stubborn, so will probably think tomorrow morning that I am worrying about nothing and that I should so it!  I don't know, I'm older now, maybe even wiser, so perhaps I will realise that this year is not a good year for me to take on this challenge.  We'll see!