Ideally when deciding to run a goal race, you find or write a training plan (with or without a coach), and then you follow it.
But maybe no training plan out there is an ideal fit and you don’t have a coach. Maybe you had a plan and found out much too late that the plan is not working for you (and because none of us can rewind time, you can’t start over!).
Of course, it is entirely possible for a runner to train for a race without following a hard-set defined training plan. It might not adequately prepare you for the race, and therein lies the risk.
But then again there’s always a non-zero chance that following a given training plan doesn’t quite prepare you for a goal race either. Any approach to training comes with its set of risks. What would be the fun and accomplishment in training for a race if any recipe or approach made doing it foolproof or easy?
Still, if you want to run a race and you have at least a couple months to generally train, you could prepare for that race without a specified written training plan. It’s as simple as a consistent habit of multiple workouts per week, with as many of them as reasonably possible being specific endurance workouts: Workouts that specifically work on things you need to do in the actual race.
It helps if you’re already running regularly and in some degree of condition to race, but even if not you could adequately train with a general, consistent schedule provided you have enough time before the race.
Again, training for a race involves executing with these acute factors: