I gain easily, I'm "blessed" with a real mofakkin' slow metabolism. I don't eat much, eat reasonably healthy, and still gain weight. Only real way to keep metabolism up is to exercise consistently. I used to run cross-country in HS and college and generally hike in summers, so my body is used to that kind of burn, and when it doesn't happen, I gain weight.

Schedule in high school: 1 mile round trip to school, on foot. 1 mile round trip to the sports fields for practice after school. 1 mile run during gym class. 3 miles run during sports practice. 1 mile walk with my mom/the dog in the evening.

Total: 7 miles.

College was pretty similar since I lived and worked off-campus.