Sick of cramps 4 yrs in a row

For the first 3 full marathons, I followed the novice plan by Higdon. In all 3 races, I had cramp in my calf muscles, but I still managed to finish at around 4h30m. My target was below 5h.

This year I switched to the Immediate I plan (http://www.halhigdon.com/marathon/inter1.html) to increase mileage, hoping that would stem onset of cramp, and targeting below 4h30m. Result was not I had planned, not by a long way. I was stopped in the last checkpoint, near Wanchai sports ground, and forced to board the dreaded bus! Although, by that time I was limping so badly I was dreading to face the crowds in Causeway Bay :) This time I develop cramp in my quads as early as 20k, and progressively got worst. Basically I walked, then limped during latter half of the race. I didn't ran enough to have cramp in my calf muscles. I ate gel every 50 mins or so, and drank a bag of sports drink at every 5km water station. What I regretted the most, was not I could not finish the race, it was I felt I had plenty of energy left in my tank, but which I could not use at all.

Can anyone tell me where I am going wrong? During training, I did my 2 longest runs (~32km) beside Tolo Highway, other long runs I did in High Island Reservoir in Sai Kung. I did not do any speedwork. After the race, it crossed my mind briefly that I don't want to do this anymore. After 18 weeks of hard work, no time did it I think it would end up like this. But a day later, I can't wait to try it next year. I don't want the memory of my last marathon to be riding a bus!