If you do very little exercise of any sort, from 6-3 months prior to skiing it would be advisable to do aerobic exercise. This is steady exercise that takes your pulse to a target level, and maintains it there for an absolute minimum of 20 minutes. To work out your target pulse: 220-your age, and take 70% of this. This should be performed ideally 3 times per week. This will help get your body used to regular exercise and is also a good way to shed a few excess pounds. However, skiing is more anaerobic, ie your pulse is up to a higher level for short bursts, and therefore your bias for the 3 months prior to skiing should be on anaerobic training. This is most easily performed with interval training. For example, spinning classes or running fast for a minute, then walk for half a minute, (repeated for 20-60 minutes, as able). Do make sure that you are well warmed-up before doing these, and if you have any health concerns seek medical advice first. If you have concerns about any of your joints, then try and stay away from running, and perform your intervals on the bike or cross-trainer. These intervals should fatigue your muscles so that they feel tired after! You therefore ideally should do those alternate days, to allow your muscles to recover in between.