Originally Posted by PeteHski

If you are on a very low carb diet and want to ride further then you need to ride slower to stay in the predominantly fat burning zone. Over time your speed should still improve, but if you go too hard you will burn through your limited glycogen store (carb based) and quickly tire. This is probably what you are experiencing at the end of your current rides. The faster you ride the more dependent you become on fuelling with carbs. If you can tolerate carbs while riding then that will help to maintain your energy levels.