I too am in a long distance relationship and my girlfriend is suffering with endo. At times we both suffer because of the pain. I wish I could help her more but cannot. I do whatever I can and support her however I can.
Communication is key.
You will need all the support you can get. Just look around on this site and there are plenty of women here to help you. There are also a few men who happen to come to this page looking for any way to help their loved one. I wish I could say something different but your relationship will be put through some serious tests. Not too many men would willingly enter into or stay in a relationship where they know that the woman will be in serious pain a lot of the time.