Personally, a long unnecessarily drawn out love vs. duty conflict annoys me just as much, because I'd start to wonder why they're even together, if neither one of them can empathize with what the other is going through. That line could work for part of it, though. It seems to me that if they are really good for each other, they'd resolve it somehow, and respect any choices the other makes.
You could probably also do the will-they-won't-they for some amount of time, but who says that the romance has to end with them getting together? Relationships grow and develop just like any character. It's not like they'll know everything about each other even after they hook up. Even then, it's not strictly necessary to keep shifting love interests, or to constantly have the 'are they cheating on me?' worry, because that's just the same stage over and over again. Take a look at Jim and Pam from The Office, if you want an example of an extended relationship story of a regular couple.
Maybe both find that they have to reconcile what they want out of life if they want to be together, because they want to do different things.
Maybe they never keep secrets from each other, but one day, one of them gets into a situation where they have to keep a secret just to protect the other one, and it starts coming between them. Maybe the secret has to do with their past.
Maybe one of them is somehow afraid that they'll bring harm to the other. A sense of protectiveness coupled with fear can go a long way.
Also, you could include many little every day moments that show how close they are, without having to shove it down the reader's throat just to remind them that they're together. They'll probably have their own jokes, and little quarrels.
If it's a series, you could very well have them start a family, inadvertently or not, which would bring a whole heap of drama on its own. Or, there could be drama between both of their families, which could affect the relationship.
You could also have one of them get dangerously sick, even if they don't die in the end. The fear both feel, and the strength they gain from dealing with it together, will make the relationship come out stronger.
Most of all, envision where you want them to be, at the end of their life together (even if that's way past the end of your series). Are they that old couple that still dances together in the evenings, even without music? Are they still travelers until death? Whatever it is, imagine it, and then picture all the things that would make them who they are at that point. Heck, maybe you could even sit down with the really old versions of your characters, and ask them for the story of their life.