I find that I can keep my fitness base through the winter with regular long runs and relatively slow pace. I add tempo (LT) runs as Spring arrives and then spend about 6-8 weeks doing Interval training for late Spring and early Summer races. Then I have another "semi-relaxed" period followed by more tempo runs to prepare for a Fall marathon followed by JRA (just running around) in the Winter.
One of the problems that I have with my own training is that when I am at my best, I am also very close to the level of activity and intensity that provokes injuries. I can only sustain this for a few weeks each year.
I am a big believer in the training methods of Arthur Lydiard.
Here is a short discussion of periodization as it applies to Cross Country running but I am sure you can see the parallels to cycling and could adapt this to almost any racing season.
Dan