When you become fat adapted it will be easier as you say. I find I generally start from a lower point energy-wise than I would do when I'm eating more carbs, but I don't get the drop off in energy because my body is already burning fat as fuel so it doesn't need to 'switch' over. I would say try reducing the intensity for the next few runs, slow down and maybe don't run as far as you would normally.

My running is probably best described as 'inconsistent', I'll have a few weeks of running 3-4 times per week and then months of not running at all. I've been low carb now for 6 months and took my exercise down to nearly nothing for the first few weeks, then introduced various things back in, getting used to doing one before I added in the next. Running is the last thing I added back in, but for the last 2 months I've been running 3 times per week quite happily on a run/walk plan. I'm not suggesting you wait 4 months before you start running again (I also do cycling, swimming, karate and a couple of strength training sessions per week so I chose to rebuild these before I concentrated on running), but be aware that it will take you some time to adapt to your new regime. Also, if running feels too hard at first maybe try a run/walk schedule, I find with the improved endurance and reduced recovery times that LCHF gives me that a 1 minute walk break feels like pressing a reset button and I start my next run interval feeling as fresh as if it's the first even when I've been running for an hour or more, it's allowed me to increase the length of my runs much faster than just running would do.

I am a daily exerciser - step aerobics 2x per week, weights or weights/cardio 2x per week, Insanity 1x per week, running 1x per week. When I started LCHF, I didn't really notice any exhaustion or performance declines in my step, cardio/weights, or Insanity classes. But running--ugh! I would struggle through the workout, then feel horrible for a few hours (often vomiting). This didn't get better until about 2 months into this WOE. Not sure what it is about running that was so much harder on me than everything else, but that was my experience.

Now, though, I'm running farther and faster than I ever have. I'm up to 11 miles at about 10:20 average per mile. And no vomiting! You'll get there (wherever "there" is for you). Just listen to your body, persevere through the next month or so, and it should be smooth sailing!