One of my favourite summertime workouts is a continuous long distance workout that incorporates several different activities, For instance, you might, start by playing soccer, then riding a road bike, then doing a run including ski walking for a total workout time of betweeen 3 and 6 hours.
The activities to choose from are pretty much up to you, but my suggestion is that seeing as this is supposed to be a ski workout, choose exercises that involve some degree of specificity.
As much as is reasonable, incorporate choices that use both upper and lower body, and at least one of them should be of a particularly ski specific nature (rollerskiing or hiking with poles, for example). Some of the most common activities to complement the ski specific ones would be paddling, road or mountain biking, swimming, and regular running. As mentioned earlier, if you are doing this in a group, a game such as soccer is great for keeping some of the clumsy people in the coordination game. Don't go at this with foolishness though-you do want to stay injury free.
So that's the workout, but you might be wondering what the purpose is. Mainly, the objective is to stress the system with a long cardiovascular workout, but to limit the toll on specific muscle groups by varying the activity. As you can imagine, a six hour combination workout will leave you feeling much less sore and tired than a six hour run. This means that you can recover more quickly from what is a very long workout and resume other hard training more quickly. Arguably, you are less likely to bring about overuse type injuries as well.
For sure, there are times when it is good to do long workouts of one activity to develop the specific muscular endurance to go along with the aerobic endurance. At this point in the year, however, you are still quite a long time away from the racing season, and the specific demands of skiing are arguably slightly less important than they would be in say, October.
From my perspective, you should still be doing a fair amount of ski specific training at this time of year, but there is enough wiggle room to allow you to mix up the workouts a bit and include some which are not totally ski specific. This workouts fits the model.
Finally, 4-6 hour workouts are long, no matter how you slice them. Changing formats during the workout does make the job easier from a mental perspective. Training effectively all year requires that you preserve a certain degree of mental freshness and this type of workout might constitute one way to accomplish that goal.
Some key points to remember:
1. Try to organize things so that you don't lose too much time in your transitions. Have the clothing and equipment that you need ready for all the activities more or less ready to go so that the workout does turn into three separate workouts of 1.5hrs.
2. Like any long workout, plan your nutrition requirements ahead of time, including what you will need to eat and drink leading up to the workout (even in the days before), during the workout, and after the workout. The transitions make it easy to re-fuel during this long workout because you can leave food, water, etc at your "transition zone".
3. Gatineau Park is a great location for such a workout. You can park at one of the many beaches or the boat launching areas. This means that you can easily incorporate water sports, or simply go for a swim afterwards.
4. Thanks to the coaches that planted the idea for this type of workout in my head: John Suuronen, Dave Mallory, Pavol Skvaridlo and John Langstone. Not to mention those triathlete people and their "brick workouts".