I recently timed myself running a 5k. I don't have any previous serious exercise experience, although in the past there have been periods when I've run a bit. My time was a bit under 30:00, and I've done it a few more time since, going as quickly as 26:50.
I also discovered, via simple googling, various race training plans. These all seem to be focused on training for a particular race in a certain number of weeks, and combine a bunch of different types of runs at varying paces, distances and intensities. I have no reason to doubt that such plan is the best way to train for a specific race, but that's not my goal -- I just want to add exercise to my life.
Would it be bad forgo such a plan and just run a 5k a few times a week? I'd like to do the simplest thing possible, because I think it will make me more likely to stick to it. I would of course reconsider, however, if a more varied routine would confer significant additional benefits or help avoid injuries.
(Why 5k? No reason in particular, it just happens to be a standard length that matches up well to the amount of time I want to spend running each session.)