There is no magic pill. I would highly recommend NOT taking steroids and instead do what has already been mentioned; pick out a workout routine and attack it! Just get out and run, or join something that gets you more active from time-to-time.
I've seen lots of out of shape people at 5ks, and that's ok! A lot of folks are getting back to the active lifestyle and they're applauded for it. Sign up for one so you have a goal. Best of luck!