First, it is quite a long walk that’s why you need to start early in the morning to get to your daily destination. Second, at a high altitude above five thousand meters time, it will blow strong wind late morning or before midday, to avoid this you need to start early morning. And the third reason; warm air has less density of molecules. Thus, when you breathe in cool/cold weather, you are getting more oxygen into your lungs. Hot weather produces the feeling of being "hard to breathe" due to the less amount of oxygen density.