It's partly from being out of shape and stress, it's also partly diet. Less energy drinks, more water and healthier foods. Make sure you're getting enough magnesium if you're prone to cramping.
The other thing is work positioning, and using the least energy possible. Use two tie in points (rope and lanyard) so you're not straining, and try to rest when your cutting. Work close to yourself, don't run your saw one handed at arms length. Two hands and working close will wear you out a lot less.
Having said that, if you're having a big day, 8-10 hours in the tree and blocking down with big saws the. You're gonna feel the burn. We've all been there.