If you have time to train for 12 weeks and can accommodate a taper week, go with the 12-week training plan. If you buy the plan and don’t like it, just send an email to email@example.com
In terms of training volume/week, the 12-week plan starts at about 5 hours and tops out at 7 hours 40 minutes in the last week.
You can still do backcountry skiing as part of your aerobic workout, just be conscious of the total workout time and progression from one week to the next.
You can develop your own plan based on the book. Do four weeks of Base and then switch to Muscular Endurance for the last eight weeks. Have a recovery week every fourth week and gradually increase the volume.
Wish you the best!