1. You Walked Everywhere
In the U.S. if you went to a state school or other major university, your college campus was likely massive. The average college student walks 10,000 steps per day, which is the equivalent to 5 miles. The average person burns roughly 90 calories per mile of walking, and when you consider the after burn, that number becomes about 110 calories per mile. Burning 400-500 calories everyday, without even noticing is nothing to sneeze at. Especially when you consider that burning those same calories in the gym today requires 45min to 1hr of intense exercise and focus.
2. You Carried Heavy Things
The average weight of a physics book is 4.8lbs. In college you may have had 4 classes in one day and lugged all four books around. That's about 20 extra pounds on your back, as you speed walked a couple miles, over the course of the day. Carrying 20lbs on your body, for long distances, meant your body required that much more energy to perform. A greater energy requirement meant more calories burned.
3. You May Have Played Co Ed Sports
Co Ed sports like soccer, touch football, and soccer were very popular in college. It was a way to stay active and meet members of the opposite sex. Not everyone leveraged Co Ed sports to find their future mate, some participated just to stay active. It was fun cardio with friends, and the fitness benefits of it were mostly an afterthought.
4. You Laughed & Had Fun
Let's face it, unless you were pursuing a medical or law degree, college was relatively low stress. All of your friends were either within walking distance or a quick car ride from you. This meant more time and opportunities to socialize and lots of laughter. The effects of laughter on the brain help reduce hormones caused by stress. Stress hormones can cause weight gain and weight retention, but the endorphins released from laughter mitigate them. Studies show that even the anticipation of laughter helps reduce stress hormones. So, by merely knowing you were going to get together with friends or have a good time made you healthier.
It may have been easier to stay active in school, but it doesn't have to stop there. Think about the times when you were in the best physical shape of your life. What activities were you participating in? Once you figure that out, try to replicate those activities now.
Let me know in the comments what you think, and what activities you participated in at school.