Yep, i had this too. I 'cheated' to solve it after discussing it with Mmoranmic, who does some awesome runs and also reading stuff from Trainer Dan.
These two use carbs during cardio.

I run for about 45 mins in the mornings, the first half of it uphill.
Before I leave the house, I have a 200ml glass of grapefruit or orange juice - 16 carbs, in a very 'instant' form.
It takes me about 10 minutes to drive to where I run.
The carbs are beginning to hit 0 - 5 mins into my run.
I checked my ketosis out afterwards and at no point during the morning was I out of ketosis.
So my reasoning is that my muscles need the carbs to work this intensely maybe as there's a limit to how fast you can oxidise fat, that the carbs are totally used up during the workout so don't affect my ketosis later.
It solved the wasted legs feeling, so that I was able to get a normal workout. Before that, my legs ached so much, I couldn't go fast enough to get a sweat on or puffing.

More in my journal and some in Mmoranmics too. She has tried hard to execise intensely on lowcarbs too and has given up, she says she needs carbs during the exercise.

All the best, hope this is some help, Jay