Bicycle spin class 3-4 times / week, year round. I use a heartrate monitor and this allows you to recognize your level of exertion and learn to control your heart rate by changing pace or resistance. It also trains your muscles to quickly flush out lactic acid buildup after you have really exerted, or gone past your Anaerobic Threshold.

In addition to, I also do training hikes with weight, 3 days/week, starting a couple months out. I am doing so currently for my upcoming GC trip. I can't come close to duplicating the terrain here in central Texas, so I train with about 50% above my expected weight.