I have the good fortune to be playing games with some of my friends for over 20 years. Recently we've started a grudge match between Tyranids and the Imperium. Namely Astra Militarum and Space Wolves vs a ton of bugs. We tend to naturally fall into multiple game grudge matches when we play our regular games. So this can easily evolve into a narrative campaign. Having a professional writer and a college professor in the ranks will hopefully allow us to develop a story line and background.
What makes a campaign last for a length of time for our group is that we don't need the six of us every time to play. Each of our collections are substantial enough to fight any size battle we may need. Some of us even have armies that can fight on either side of the campaign.
The devil is in the details and book keeping so the next step is a briefing of our past conflicts and background of our forces and commanders.
The main thing to know right now is that this winged A-hole has been kick'in butt the last couple of games.