doncalypso, you probably don't understand how it can survive the test of time because you've never done it. It works because you make it work. It's hard but it shows each other that you're loyal and faithful, and it really is a test of your love for that person.
Now personally, I've been in my relationship for nearly a year. Nine months of that will have been long distance. I get to see him every two weeks usually, and trust me, it gets tough. Lately yeah, I've been in nonstop fights with him. We get on each other's nerves because when we're away from each other we lose sight of who we love so much (and our personalities kind of change from lovey-dovey to no-nonsense schooltime). I've had my doubts this will work; so has he.
But in the end...we always end up still together and stronger because of all we've had to go through.