I cannot go to the gym without bringing my mp3 player. Without it is like going out of the house without wearing your underwear. It is that important. That is why it has to be tucked in somewhere in my backpack. Always.
Now, what do I listen to when I hit the treadmill, and is running in a 10km/hr speed? Lots of Groove Armada, Daft Punk, and Fatboy Slim. Their music keeps me upbeat and alert. And I can stay focus and stretch my endurance more when I listen to an upbeat song (as opposed to hearing what the gym sound system plays).
In a recent study by Dr Costas Karageorghis, from Brunel University, listening to music can boost endurance by an average of 15%. The full report on the study here.