I think the answer to your failing to reach your goal at MCM yesterday is simple....you tried to run too fast.

I don't mean that you weren't adequately trained to run a 3:30 marathon.....I don't know the details of your training and marathon time predictors. However, my guess is that you fell into a trap that causes many marathoners, including lots of those at MCM yesterday, to have a performance that was less than desired....you didn't adjust your race goal and execution to account for unfavorable weather conditions.

Just read through today's many posts reporting on yesterday's MCM.  One after another talks about coming up short relative to his/her race time goal.  I have yet to read one that says that his/her time goal was met or exceeded.  For several, early race congestion was a problem.  However, virtually all report significant slowing in the last few miles....even those who were forced to start very conservatively in the first few miles due to congestion.

Race goals, and associated pacing plans, should have been adjusted by at least 30-45 sec/mile to allow for the hot day....even more for those planning to be out there 4-5 hours.  I have not read a single report among the 8-10 on this forum that says that s/he did that.  It is a very difficult thing to discipline yourself to do.  It is very hard to accept that, after you train hard for 16-18 weeks or longer to reach a certain target time, you should ease your goal by 10-30 minutes or more on race day to account for a little warm weather.  But, that's the way it is.  Don't adjust and you just ask for a much less than optimum race and a big disappointment.

The most common cause of cramps is muscle fatigue.  That can occur in a marathon if you are inadequately trained for the distance; if you run too fast for your level of training; if you go out too fast early in the race to try to "bank time"; or if you fail to adjust your pace to account for less than ideal race day course and/or weather conditions.  I suspect that your problem yesterday was the latter.

All of the pre-race methods used to "predict" a marathon time and set goals assume that conditions will be good on race day.  However, when they turn out to be not good, then goals and race plans should be adjusted.  To not do so just invites cramps and/or a death march in the last few miles.

Unlike some others, I do believe that a shortage of electrolytes in a marathon, particularly a hot one, can contribute to cramps.  That might not be a primary cause, but I do believe that it can be a contributing factor.  I think that you made a mistake in not taking the Ultima every time it was available.  Don't worry about the bad press about Ultima on this forum.  It won't hurt you and it just might have helped a little.  However, by not adjusting your race goal and pacing, using the Ultima probably would not have enabled you to avoid the cramps.

I also think that the tendency to cramp during a marathon will abate with subsequent training cycles.  In other words, the more you go through training cycles and marathon races, the better your body adapts to deal with the stress of the 26.2 mile race experience.

I think that you did exceptionally well to come within 13 minutes of your race goal under the weather conditions and considering the cramp problems that you had.  The fact that you said that you "felt pretty good overall" is significant.  That indicates that you certainly had a faster marathon in you....I suspect several minutes faster than 3:30.  You were simply a victim of circumstances.

Congratulations on an excellent first marathon and good learning experience!