I find that there's an intensity sweet spot. Usually when you're just starting to work out "for real" you just can't help being worn out the first several times

I find that there's an intensity sweet spot. Usually when you're just starting to work out "for real" you just can't help being worn out the first several times. But soon after the work out actually makes you more energized. If being worn out persists I would reduce intensity and perhaps tweak the weights/aerobic balance. Maybe you need more running and the like, or perhaps more weight type exercise. A lot of "natural" physical activities (i.e. manual labor) tend to be repetitive in ways that don't exercise your body evenly enough, which can cause greater fatigue.