How to train for Birkie?

The next thing on my list of thinks to do is the Birke. I know I won't get to it this year but I want to try and get some longer ski races in. How often do you ski and how far? I have roller skis and will be getting them out after my 1/2 marathon on the 20th of this month. Does running fitness help for skiing? Anyone have training plans, how about birkierunner?

Thanks