Your body is just yelling at you to eat more potassium. Unfortunately, a lot of foods that are high in potassium also happen to be high in carbs -- so your best bet is eating a cup of Spinach a day, which is over 1/2 higher than the potassium found in a banana. Our bodies require about 2,000 mg of Potassium a day, and dieting AND exercise both lower potassium levels (which isn't helped by the fact that a lot of multi-vitamins don't include potassium at all...)
Also be sure to really stretch your legs before and after you work out, that'll become more and more important as your runs get longer.
Hope that helps! Keep up the great work!