Sounds simple, dumb, but just start running, cardio workouts, simple push-ups and sit ups. You'll definitely notice a big difference.

As you may know, I worked out all the time and was continually playing sports in HS and first year of college. I stopped for a year, but then I started working out again. I would run over a mile and do light weigh workouts. You can do A LOT with just two dumb bells. From squats, sit-ups, to dumb bell presses, etc.

It feels so good to exercise. If that all fails, just play basketball or something.

Just remember drinking doesn't help much, and you'll sweat a hell of a lot more.