Long distance hardly ever works. Either one party cheats or lose interest, or both just get fed up with not being able to see each other as often as the couples who are near each other. I have seen too many cases just within my immediate circle of friends, the guy got a fellowship to go to another state to finish his Ph.D., he broke up with her just so that there would not be an opportunity for mistakes to develop.
However, I have seen a case where the man asked my friend to marry him. He used a marriage vow as a seal. I say, that s smart! They re still together to this day and will be marrying soon!