I finally walked my 20-mile endurance walk prior to the marathon. I'm relieved that I did it and I am not suffering from it, either. I went to the doctor for a follow-up appointment this morning and he seemed really ok with the fact that I did it. He didn't lecture me (he has in the past). He gave me some tips and he answered my questions and he put me on prescription strengtht Naproxen until the marathon is over. The first 7 miles went extremely well. The next 7 went well, the next 2-3 were just ok. The final miles were painful. I tried to imagine myself walking about 6.2 miles. I couldn't imagine it.
The doctor did say it was ok with him if I wanted to try another 20-mile walk to see how things go. The issue as I see it is that my overall pace was 22 minutes/hour. That is awful and it will not get me through the marathon.