I tend to agree with jschwab. I'm walking my first half marathon in about 4 weeks. I tried going low/no carb for my 9 mile walk last week, and not only bonked, but got dehydrated as well.

Tomorrow I have a 12 mile walk, and I've decided that I'm going to apply sort of the "carb cycling" principle to my remaining workouts as I train for my half. I'm going to cut out the sugary drinks, opting for water and electrolyte pills instead. But I have a fuel routine with Shot Bloks that has served me perfectly well through two Century rides and the first two months of my half marathon training. I'm going to continue with that.

I asked my counselor (as I go to Medi Weightloss) about it this past week. As I put it to her: "I burn 400 calories an hour - about 100 per mile - during my walks. My body is used to consuming 100 calories of carbs each hour. So, if I consume 100 calories of carbs, but burn 400 calories, will that throw me out of ketosis?"

Her response was, "It's pretty clear to me you know your body's needs when you're working out. I can't tell you for sure, but do what's working for you and what you need to do to keep going for a long period of time."

So I'm giving it a whirl tomorrow and will be checking to see if it throws me out of ketosis for a few days after. I'll report back.