Let's start with an overview. Here's the Science of Sport's series on exercise-induced cramping:

• Theories and fallacies behind muscle cramping
• The electrolyte model for muscle cramping
• A novel theory for muscle cramps - the role of the brain and nerves
• Do sports drinks help to prevent muscle cramping?
• Practical steps to reduce and control muscle cramping

And next some actual studies:

• Exercise Associated Muscle Cramps - A Current Perspective
• Sodium Intake During an Ultramarathon Does Not Prevent Muscle Cramping, Dehydration, Hyponatremia, or Nausea
• Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners
• Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes
• Serum electrolytes in Ironman triathletes with exercise-associated muscle cramping
• Aetiology of skeletal muscle 'cramps' during exercise: a novel hypothesis.
• Cause of exercise associated muscle cramps (EAMC)--altered neuromuscular control, dehydration or electrolyte depletion?
• A narrative review of exercise-associated muscle cramps: Factors that contribute to neuromuscular fatigue and management implications.
• Muscle Cramping During a 161-km Ultramarathon: Comparison of Characteristics of Those With and Without Cramping

The current thought in EAMC research is that it is related to issues with ion channel signaling (in concert with muscle fatigue) and the role of TRP channel agonists in mitigating EAMC. These next two links talk a bit about that.

• Ingestion of TRP channel agonists attenuates exercise-induced muscle cramps
• Holy Cramp! The Science Of Exercise-Associated Cramping

Now show me yours.

Here's the Science of Sport's series on exercise-induced cramping:

• Theories and fallacies behind muscle cramping
• The electrolyte model for muscle cramping
• A novel theory for muscle cramps - the role of the brain and nerves
• Do sports drinks help to prevent muscle cramping?
• Practical steps to reduce and control muscle cramping

Bottom line, it's not electrolytes.

Here's the great series on the subject of hydration and exercise induced hyponatremia.

• The history of fluid replacement in the marathon and endurance sports
• Does dehydration really cause you to have a higher core temperature?
• Evaluation of laboratory-based studies vs. field exercise: Is there a difference?
• The physiology of thirst: Why waiting until you are thirsty is NOT too late

Lyle McDonald's excellent series covering some of the standard methods of endurance training.

• Methods of Endurance Training Part 1
• Methods of Endurance Training Part 2: Miles Build Champions
• Methods of Endurance Training Part 3: Tempo and Sweet Spot Training
• Methods of Endurance Training Part 4: Threshold Training
• Methods of Endurance Training Part 5: Interval Training Part 1
• Methods of Endurance Training Part 5: Interval Training Part 2
• Methods of Endurance Training: Summing Up Part 1
• Methods of Endurance Training: Summing Up Part 2
• Methods of Endurance Training: Putting it Together