It's really easy to be jealous or in awe of people who seem to stay thin and fit naturally. While it's true that some people have height on their side or a slightly faster metabolism, if you're over age 25, most of the thin, fit people in your life work really hard to stay that way (whether you realize it or not). Instead of dwelling or complaining about how thin and awesome those people are, pick their brains! Ask them what they do each day to stay in such good shape. You will probably be surprised to find that, even though you rarely hear them talking about it, these people are spending hours each week being active and keeping a decently close eye on what they eat. Instead of thinking you can never be as committed as they are, use them for support in your fitness goals!
