Ok - without goin into type1 and type 2 fibre training systems, what every one has missed is your hydration levels - prior to kiteing...
You mention missed landings and slower reactions - 2% loss of body fluid through exertion, urine & excretion makes the brain tired and lethargic, minor cramps, sound familiar..
4-6% you'll be start forgetting where you are and turn into a blithering severely dehydrated idiot.
And about 8% you'll wake up Dead
You can train motor units to fire till your hearts content but your brain will effectively shut them down to sustain itself..
'Oh - but I don't feel thirsty' you say ? Because your brain is more clever than yourself lol it can make you hungry to absorb water from the food.
Who's heard of the saying, if you feel hungry drink some water first ??
You can set yourself up for dehydration in the days prior to any exercise - a few beers wine coffee the list goes on of food that will make you loose water... Or consecutive days training without topping up your electrolyte levels.
So on the big day - you can be border line hydrated etc
With fluid loss, comes electrolyte loss as the body automatically flushes its levels to maintain a balance in your system... You may know of the story behind Gatorade or seen marathon runners hooked up to a drip - because it wasn't just water they lost. Ahaa now we're onto it
The picture pretty much tells its own story about who in the "food chain" is in charge !
To maximize your endurance performance make sure you are well hydrated "before" goin out - what I do is buy some electrolyte powder and mix it up and drink a few glasses the night before, then take some to drink - making sure I have little breaks and take a banana or two for the energy hit.
Also remember to add in a good stretch or flexibility routine daily...
The old broom stick twists are a great for trunk rotation flexibility in kiteing.
Remember - you are only as young as your spine is Supple