Hi Ned,
Skiing is a great substitute for zone 2 and endurance workouts. For the endurance rides, I recommend, most days, doing 30-60 mins indoors on the bike, then finishing off the balance of the time outside doing some fun skiing. Hiking can substitute if you are hiking fast enough to keep your HR in the zone the workout calls for. Cross training is super during the Base plan to keep the fun and outside time in your week when riding is unsafe outside.
Coach Lynda