A couple of seasons ago, to get ready for a Harb ski camp, I worked with a personal trainer at a pricey downtown club and spent 3 months doing a bunch of different exercises, including:
- treadmill work
- balance ball exercises
- weight machines and free weights, and
- various body weight exercises.
Then last year, other than a little inline skating, I didn't do anything except for intervals of kettlebell swings -- 10 reps, 30-second rest intervals between each set, 5 sets. Took a grand total of 5 minutes (okay, ten with warmup) three times a week. Also 3 months, and was in way better shape for skiing.
There's been lots of research in the last few years about how intervals are the most efficient way to simultaneously build both aerobic and anerobic capacity (eg, Google "Tabata Protocol").