Usually I hit a wall not because my legs or cardio are tuckered, it's because my butt hurts or my hands or any combination of the two. Balance seems to be the key for me. The harder I pedal, the more weight I transfer to the pedals evenly distributing the weight between bars, saddle and pedals. This relieves the wrists from carrying the brunt of the weight. Riding loose as posted above, more aero with elbows bent at 90 degrees and a light grip on the bar get me further faster. It's a lot more pleasurable but does require a bit of ab work and back trengthening exercises.