Taking It Too Easy
Your go-to 2.5-mile jog is better than sitting on the couch, but unless you build on it, you're standing in one place. "One of the basic tenets of exercise physiology is the principle of overload," explains Stephen McGregor, director of the exercise-physiology lab at Eastern Michigan University. "This states that you can improve only by working a little harder than you have in the past."

The Fix: Ramp it up. Slowly. Runners and cyclists: Start your interval routine with eight one-minute intervals at the fastest speed you can maintain (take two-minute recoveries in between). In each subsequent workout, do as many intervals as you can at the same speed until you can do 12, then increase your speed so that once again you can do only eight intervals. Just don't overdo it.