People are insane if they think "realistically they wont remain together because of distance"
1: Its a manga. It doesnt have to be "realistic".
2: We live in a time with easy communication and face chat. They can literally "see" each other all the time.
3: Their bond goes beyond what a lot of young couples would be. Many young "normal" couples (so just dating in HS for fun, etc) don't have the same situation these two have had, not that a more normal young couple is wrong really(its good to live/learn/grow/etc) but there's extra layers here that many couples wouldn't have that would make them remaining together far more likely.
4: Waluigi.