You definitely need to give yourself some time get fat-adapted. My first two-three weeks of paelo were really crappy. I had no energy in workouts (I should have taken those weeks off, but I didn't know then). Even simple warm-ups got me crazy out-of-breath and weak. But in the middle of that third week, it was like a switch flipped and I had endless power and speed. It's crazy. I don't do much in the way of endurance training, I'm a believer in the short-duration, HIIT method. However, I do endurance events (mostly 24+ hour running races, but long-distance cycling too) just to prove that I can still do them (with no training!) and actually finish pretty well. For those, I do not carb up or anything because I've become so good at being fat-adapted that I'd rather just keep it dialed in. An added benefit is burning fat brings it's own water, so you don't have the extra hydration requirements as you do when burning carbs, so that's less water to carry. My last experience was a 10K race done in a fasted state (it was a pretty early start and I didn't want to grab breakfast). I was about 3 minutes faster than I was the year before; and, it was a constant speed. I had this constant low-level delivery of energy. I couldn't sprint or pick up the pace at all, but I was able to maintain a fast constant pace for all 10K and when I finished, I wasn't tired at all, I could have done another 10K.

Just to be clear, I'm not advocating endurance exercise being good for you. And if you follow my answers here, you'll know I'm against it. The only reason I do any races (again with NO, ZERO!, training for them) is to prove my "general badassery" with regards to my HIIT being able to transfer into other things. I know it's bad for me, I just do it for ego.