The power and importance of building a good base at the start of any training plan can not be over-stated. None of the training that happens after base building will be as effective if the base is not secure, like the very foundation of a building. Weak or shallow foundation, and the entire structure is suspect.
This is why the winter makes the best time to do this - no distractions of competition or competing interests outside (except for some ski slope time now and then), and so it is the perfect time to build that base. The beauty of Winter Training is that no matter what sport you engage in, if it has an aerobic component, it requires base building and this is where Winter Training shines. Winter Training will not only make you better for the Spring and Summer that follows it, but year after year you can expect to get stronger and stronger, fitter and fitter.