In reply to The Ice Doctor:
Carry less on your back, you can't pack experience as the saying goes.
Frequency, train a little midweek, rather than blow outs at weekends. Pacing, sweat management, decent food, sleep and hydration the night before etc.. Personally unless it's a big day, 10hrs plus, I don't tend to eat much, other than a few snacks during the day, but I'll always have a litres of water with me. You are working aerobically generally, or very close to the threshold, so if you are hydrate etc. there is no reason why your body can't convert energy itself, you should not need large amounts of calories on the hill. Better to consume pretty quick after to aid recovery.
Take the sky cycling team approach. They target every area of their athletes, even if each one individually might only yield a 0.5% improvement in performance.
Post edited at 12:43