Do the things that interest you and do them with all your heart. Don't be concerned about whether people are watching you or criticizing you. The chances are that they aren't paying attention to you.
Every time you have a carrot instead of a cookie, every time you go to the gym instead of going to the movies, that's a costly investment in your health. But how much you want to invest is going to depend on how much longer you expect to live in the future, even if you don't make those investments.