Originally Posted by Carbonfiberboy

Undertrained/overworked. Simple. There is no proven connection between cramping and either hydration or electrolytes in spite of considerable anecdotal testimony. However there is a well known and well proven connection between cramping and working at over your training level. If you're doing 150 miles/week or so, then it's almost certainly insufficient high end work: hill repeats, LT speed work on the flat and the like. A very simple workout on the flat is 5 reps X 5' work X 1' easy at LT effort or so.

I would not suspect either fit or cadence. Since you cramped both sides of your legs (perfect), your pedaling style is also not at fault. I am more of a seated climber who does not do well at long distance if I stand and pop over stuff. However there are many people who do, and it sounds like you might be one of them. One thing you might not have done is to stand and pedal more frequently when not going hard. I try to get up about every 10 minutes and stretch my legs at a low cadence. However, in this case I don't think that would have had much of an effect.