It is most likey dehyration. Make sure you are drinking enough all the time Not just during your long run.
I have had these same issues too. This is what I did to resolve them. First I drank enought water. second I stopped drinking coffee the morning of the LR. Two days before I ate light. Think Brat diet. Bannanas, rice, applesauce and toast. Not only are these foods easy on the tummy but they are great fuels for the long run. Also two days before I start to watch how much protein I take in.
no greasy foods, no sugar, no heavy protein. Make sure you "go" before you go.
After awhile your body gets used to running longer so I added one thing back into my diet at a time.
don't give up. give yourself time and your body will listen to what you are telling it to do.
try these things first. you really can't break up a long run into days. then it is not a long run. don't worry about what you missed. try again.