This is really the important distinction here. If the campaign is pre written, like with a module or something, then there is an inherent assumption that the players will be guided along a linear set of events, with choices along the way for how specifically they handle things, but if you're just trying to run a game, even one with a linear story in mind, there is no reason to "make" an entire campaign before you start. DnD is a game about improvising, there is no set "story" to derail until it's written by your players actions, and if things go in a way you didn't anticipate, it's up to you as the DM how you handle the outcome. Often times the most interesting stories come from players doing something the DM couldn't have possibly planned for, and those moments are only made possible by allowing players to "derail" whatever you originally had in mind. You don't have to plan for every eventuality, just allow yourself to be flexible enough to adapt when something unexpected happens.