In the FASTER study by Jeff Volek with high endurance athletes, it took about 6 months for them to reach their peak in terms of quickly utilizing fat stores for immediate energy.

I don't have any personal perspective. I capped out at 11 miles for my longest ever run. After seeing repeated references to endurance athletics being linked to elevated BG, I have shifted back to more strength training. I never liked running. I only did it because I thought it was supposed to be good for me. Since I don't have any weight to lose and my only objective is further lowering my BG, my personal training has shifted to the two types that have been shown to help - strength and HIIT.

When I was running, I had no issues once I got my electrolytes worked out. Of course, I was not going as far or as long as you, so...

How about sodium?
I'm no athlete by any means, but I've definitely read about others needing to replenish sodium during the run.
I think there are a number of ways to go from things I've read and heard from others over the years. If you're determined to not use carbs, you may want to look into ketone esters or MCT oil for a boost during. Of course you need to be aware of your MCT tolerance because it wouldn't be good to get the runs on your run....
It also may not be a problem to strategically use carbs for your purpose. I've seen that pure glucose in gel or tablet form is often used.
There was a guy here a while back that would eat a snickers bar before long intense bike rides.
I'm sure it can be done without carbs, but I imagine you'll have to be fully adapted and also figure out your person sodium needs.

AlexandraCarlyle wrote: »

@cstehansen , that car (your avatar) looks as if it ran into the Hulk, while HE was marathon running.... yours....?!

That happened last Monday to my car. My fault. I didn't see a car as I pulled out. Other car was going about 45 mph and took off the front end of my car. Fortunately no significant injuries but both cars were totaled. My first significant accident. Only very minor fender bender before this years ago. Hopefully my last as well.

codename_steve wrote: »

So I know that on a LCHF diet, your body utilizes body fat for fuel when calories are too low to completely fulfill your needs. My question is: does this still apply under intense training (I'm about three weeks from my first marathon of the season)? My runs have been suffering lately from lack of energy and difficulty breathing (it's like my lungs are too exhausted to work). Could this possibly be that I'm not eating enough? I try to eat when hungry and stop when I'm full (not calorie counting at the moment).

I have a half marathon coming up in about 4 weeks and have also been struggling with the same issues you mention. So this week I upped my calories by 200 a day. And this morning for my 8 mile run/walk I had a serving of prunes (100 cals, 26g carbs) just before I headed out the door, and then about half way through I had a chia gel pack (100 cals, 22g carbs). It made such a huge difference. I mean night and day huge. Suddenly I could run for much, much longer distances instead of only the short spurts that was the best I'd been doing lately. And my breathing was so much easier as well. I'm going to try it again for my long run this weekend to see if I get similar results.

I hope you figure out something to work for you. But if you suspect your calories are too low, then I'd start with that.