Did you have salty residue on your face?
You need to increase your sodium and electrolytes by drinking a sports drink prior, during and after the workout. Be sure you are using nutrition that has electrolytes (all gu, gels, beans and sports gummies will). Consider taking electrolyte tablets the morning of and during your long workout when temperatures are more than 65 degrees.
Did you feel really thirsty at the end of the run or walk?
You were dehydrated. Dehydration starts with consumption the day BEFORE a long run. Make a habit of carrying a stainless steel water bottle with you the day before your long workout so you will be drinking plenty of H2O all day. While on the long run/walk, take water at every water stop, even if you do not feel thirsty. Dehydration sets in before you realize it and then it is too late. Atleast ...