One of the things that I will be writing about in my columns for TGO (a UK hillwalking magazine) is pace in hillwalking.
I've already mentioned the different metabolic / energy pathways. It is all really simplified but in basic terms you either burn fat - virtually unlimited - or sugar, which is in limited supply. Sugar needs more oxygen and burns dirty, producing lactic acid. (I know this is all simplified too much). In terms of endurance you need to stay aerobic / fat burning. As you work harder the fat burning pathway can;t keep up - you do not take in enough oxygen, so you switch to sugar burning / anaerobic. You will not be able to go for long, since you will run out of fuel and the muck produced by burning sugar will stop you. Anaerobic - intense intervals etc - are good at making you more efficient and fitter, better at burning fat but they are for training, not long efforts.
So it is about pace. Use a pace at which you can stay in the aerobic zone, burning fat not sugar. you can use heart rate measures or respiratory quotient, but one simple way to assess where you are is via the "talk test". Exercising at a pace in which you can hold a conversation is an indicator that you are still burning fat not sugar.
There is another idea though, that of frowning. I saw this study which says that frowning is an indicator of exercise intensity. I wonder if there is any correlation between the onset of frowning and the aerobic threshold?