This is just an echo of the prev. post. Your body eventually will adjust, some people take longer than others. It will be especially good for the kind of exercise you're doing, long low intensity.
I used to have the same feeling, dragging completely spent muscles. I love to hike, especially long hard hikes and now sticking to low carb actually makes me feel more energetic and strong than if I'm munching on carbs before hand.
Have faith! You should start feeling better in a matter of weeks, after months it will be great.
Meg


ps: I don't know if this has any relevance to your situation, but I find if I eat carbs in the morning I burn them off later and feel ok. If I eat them in the evening, the following morning I am really sluggish.