>> How long have you given zero carb time to adapt in the past? 

Boy, I think the longest is 2, maybe 2-1/2  weeks. I inevitably gave up because 2 weeks is long enough to put up with "hitting the wall".

I have no trouble adjusting to "low" carb (say, 50-100g per day). I do my morning ride to work on an empty stomach and have no energy loss whatsoever. But when I do meat/fat only for a couple days I start getting this rubbery legs feeling and general loss of endurance.

Funny thing - I experienced the exact same symptoms when I (long ago) experimented with natural hygiene and vegetarianism. I guess I'm just a born omnivore.