Between now and next March I'd suggest you concentrate on aerobic conditioning -- spinning, cycling, running, rowing, swimming (though not recommended as part of rowing in January...).

Starting in the spring, it's time to start conditioning your saddle interface. Progressively longer rides, preferably twice on weekends, to get you used to long hours in the saddle. When you're riding back-to-back 40-50 mile rides, start adding some weight in panniers (or a trailer) to get used to that.

You could do worse than following one of those "ride a century in three months" training plans if you want the structure.