This happened to me when I first started commuting day after day on a 20 mile round trip....thought I was invincible! Over time the body adapts to recover faster and become fitter. As other people have said, it is much to do with what you eat and your current fitness level.
Best advice I have is eat well (good balance of carbs proteins and fats) and listen to your body I.e. Take a break if you feel worn down. Over time you will adapt.