You don't really need a lot of willpower to run 5k. You just need to not try to do it immediately when you first start running. Start by just walking at a brisk pace for a couple of weeks, then start running at a very moderate pace for a minute at a time with walk breaks in between. Get a fitness watch to make sure your heart rate is at a constant level in the aerobic zone (lots of fitness watches can indicate where this is based on your physical attributes) while you run. Gradually increase the duration of the run intervals and shorten the walk breaks each week until you run continuously. It's not hard at all if you just do it very gradually. No willpower required.