I've only done five marathons so you can take this for what its worth.
First marathon Cleveland April 2004. Used Higdon's beginner marathon program. Mostly easy running; a couple twenties. Mileage around 40-45/week. Severe hamstring cramp at around 18 miles (after knocking off five 8:05 miles...tried to make BQ by big negative split...typical rookie mistake)
Second marathon Marine Corp Oct 2004 Used Higdon's intermediate program..no real speed work or long temp stuff..maybe some MP work but I don't think I followed that strictly enough. Race day eighty degree and humid. First ten miles under 80 minutes. Both quads cramped around mile 14. Limped home in 4:30, very dehydrated.
Last two Marathon in spring and fall 2005 Followed Pfitzinger Advanced marathoning 55 miles/18 week program. Stayed with schedule except for a long LT run or two (due to weather & other factors) Did the repeats of a mile, 1000m and 600m at 5k pace, ten milers at just under MP, just about everything. It was tough for me and I was able to improve my Cleveland time by five minute each time.
To me, fluid and nutrition are race day factors. A local tri-guy asked me about the Flying Pig I just ran and said he thought I cramped because of 'possible electrolyte issues' after doing his calculations of caloric input/output etc., etc. But I think getting your body used to the stress of the pace over the distance is more critical. Marketers of 'sports nutrition' products will probably disagree.