I've noticed on my two 20 milers that I have done, I have been cramping in only my right calf. I drink lots of water through out the day (i have a 34oz mug that I fill 3-4 times a day). I used to get this type of cramp at night once in awhile, but now it occurs while riding. I eat a good meal atleast 5 hours prior to riding an usually eat a banana right before leaving. I don't take a multivitamin, but thinking I should.