level of self-knowledge, honesty, openness, and relationship experience of both members of the relationship is a huge factor. lots of all the above and long-distance can work. weak in any category is deadly, and weak in several and you might as well just break it off asap and save the soon-to-be-wasted time.
i've been in one for a full year now. we met at a wedding...my best friend married her best friend (yeah, cute). i'm 30, and have been married (young and very dumb choice) and had a number of long-term and casual relationships. she's 25 and has had very few relationships. we spent this summer together, talk nearly every single day (or at least IM and play some yahoo games, lol), and see each other around once a month for a weekend. i'm in my last year of law school, and she's graduating from med school in may.
it has been painful at times, incredibly tough this summer when we were together compressing a year of relationship into a couple of months...it got bad for a while and then we sort of hit a point where we both recognized that it was going to take work, and we both now know what that actually MEANS (always heard it said in the past, but never understood it). we worked things out, and understand each other MUCH better after that time together. i'd never have been able to say i want to marry her if i hadn't spent that time with her. now i know i want to marry her, but i still want time living with her before making that kind of committment, and she wants the same. in the mean time, we are both incredibly busy (i'm writing articles for the law journal, president of the student bar association, and have more credits that the ABA would like to think are possible to take at once, plus doing the full-time job thing, and she's doing her sub-i and has a nice line-up of crazy rotations planned, particularly the two at the two hospitals we're hoping she'll match at...), and we both make it a point to clear up our time for each other...it's tough like hell to find that balance between clearing up enough time to keep things good and strong but not so much that resentment or anxiety about unfinished work gets created, but we are at that equilibrium point and have been for a bit.
it isn't all roses, is part of my point, but it isn't strictly impossible, either. it's just more crap than a sane person would want to put themself through. of course, love does that...makes you insane...
best advice? follow your heart. if you aren't that in love, then you'll figure it when when they dump you or you dump them. if you are really in love, then things will come together.
good luck!