Whether it be for running, swimming, biking, etc, I've heard over and over that intervals are so much better for building up your endurance than long distance activity at a steady pace. There's something about a high intensity workout that gets your blood flowing and heart pumping! Recently, I found the coolest website called The Daily HIIT (High Intensity Interval Training) that has a new workout uploaded everyday to burn fat and calories. Each workout is 12 minutes or less but really intense. In addition, I love their challenges like the 21 day boot camp and the 7 day challenge for a little bit of extra motivation. And the best part? You can do them from anywhere, perfect when you're traveling or in your college dorm or apartment! It's totally free and great if you are short on time but still want to sneak in a workout.
Definitely check out Daily HIIT if you're in need of a quick but rewarding workout!