*Floating the Shenandoah in the Winter Can Be a Nice Break, Even without the Fish*
When fly fishing for musky, casting is a very important aspect. Because musky flies are large, heavy, and not necessarily aerodynamic, casting requires a bit of practice and technique. Poor technique will result in a sore back and shoulder, and less casts for the fish that requires 10,000. The best technique for casting musky flies is called the "double haul." This method involves pulling quickly on the fly line with the non-casting hand during the "loading" while casting with the rod hand. This helps to generate more power and distance in the cast. To try to do a double haul, begin by lifting the rod tip high, then quickly back and forward while pulling the fly line with the off hand. I'd recommend a lighter rod and a streamer to practice this before hitting the open water with huge flies.
Another important aspect of casting for musky is using a heavy duty rod with a fast action. A fast-action rod has a stiffer tip and will allow you to generate more power in your cast as well as make more accurate presentations. “Roll casts” can be used to make a cast when space is limited or when you need to avoid making too much noise, however, this is difficult with heavier flies. Many fishers I have gone with use an almost upward cast so flies are basically side-armed out. Noise usually isn't a problem when casting for musky, but you definitely want to get as much distance as possible.
Lastly I'd add that if you are concerned about casting all day it is more important that you pick you spots. One or two good casts are better than 10 crappy ones. While musky aren't the most spooky fish, hitting one on the head instead of in front will determine whether the fish follows or goes away. Don't wear yourself out if you know that is going to be an issue. Perhaps even a preemptive Advil might be in the cards. If the casting doesn't make you tired fighting a 50 pound fish will. Just make sure you set it hard!