A training plan, however, shouldn't last too long. Depending on the length of the race, 20 weeks is usually tops. That's still a long time, but what do you do when you're not training for something specific?
First, I'd say take some time to recover. You shouldn't be training intensely every week, and it's not a bad idea to take a month or two where you run much less than normal. Or, you can spend a season focusing on another sport like cycling or swimming to help balance out your muscles. I also like to take a solid week or two off completely at least once per year--better to take a planned rest than be forced to rest because of injury.
Another option is to build a big base. There's a couple ways to do this--you can slowly increase your mileage until you're at a level at or slightly above the highest mileage your next training plan calls for. You can also use the Lydiard method. The 28 week plan (a notable exception to the 20 week limit), "Training the Lydiard Way: 28 Weeks to a PR" calls for 12 weeks of base training.
Besides recovery and base building, there's also the option of trying something like an endless season plan. In "Always Ready to Race," the author shows how racing throughout the year and training at about 80% capacity can lead to some fast races and fitness gains.
There's no right answer here--do something that works for you, preferably something you enjoy. Just don't train hard year round unless you're a fan of getting injured or burning out.