Well, skiing is an athletic activity, but if your thighs get a lot more tired in powder than on typical groomed conditions, there is very likely something going on in your technique. We've had a few good discussions of powder technique and tactics over the years at EpicSki. Click on the following link for one of the best:
Reluctance in Powder?
And you might find a solution--or at least an inspiration--in this video clip that I put together a few seasons back. It features some top instructors from around the country, as well as a number of EpicSki's own Bears filmed at various EpicSki Academies. Some of the video is a bit grainy. (But what do you expect? It's hard to get good video of a powder day, because it's just too much fun to stop and shoot video!) Brief instructional tips appear at the end of the clip:
Relax. (Relax Hard!) Float. Glide. Guide, don't twist. Balance--don't lean back (or forward). Move a lot, and move always. Speed is your friend--to a point.
To get a good taste for powder... you've got to eat some!
Best regards,
Bob Barnes