My long runs prior to that 'thon were part of the problem. My last decent long run was 6.5 weeks before the 'thon. A 26 or 27 miler at a decent pace. I finished a 50K 2.5 weeks out, but walked a tremendous amount of the last 11 miles. I had developed a severe breathing problem a week before the 50K and took several days off from running.
I ran a very slow 21 miler yesterday with no food since the night before. Worked real well. Was able to do a 6.2 mile training run this morning at an acceptable pace and my right heel feels much better than yesterday.
Very happy with these new developments. Thanks to both Vladimir and HRE.
The experiment continues.