I'm in ketosis. I can tell when the glycogen in my liver depletes. I hit a wall and can't keep going. I find that it has to do with intensity - for low intensity exercise, I can go forever. When I have to exert more (like running up an SF hill), it depletes me more.

To fix this, you more or less have to keep intensity lower. I think that's why it works well for bicyclists who can change gears.

Good news is that, after endurance exercise, you can replenish glycogen stores - meaning you can have more sugar than normal without leaving ketosis.