Q: You mention in your "Fitness Rules of Thumb" that if weight training at the same time, limit cardio to under 2 hours per week and if not weight training at the same time, limit cardio to under 7 hours per week. What is the reasoning behind this? I typically run 30 minutes/day mon-fri and lift following my run (hitting different muscle groups each day). I just want to make sure I'm not doing more harm than good.

A: The reason for limiting cardio more while weight training is to give your body time to recover and prevent overtraining. Overtraining occurs when your body does not have enough time (or possibly enough nutrients) to fully recover between workouts so each additional workout beats up your body more and more. If continued long enough this leads to all sorts of injuries, poor immune response, and subpar progress, whether in gaining strength, building muscle, or losing fat. Heavy whole-body weight training puts more stress on your body than most cardio (which is why it's so effective in raising metabolism -- that's your body's working overtime to repair muscle damage) so I recommend limiting additional physical stresses, such as cardio, while weight training to prevent overtraining.

Of course different people can have very different recuperative abilities. Elite athletes can put in several hours of intense physical exercise daily and still benefit from it. To a large degree this is true of young people in general. But for a quick rule of thumb, up to 2 hours/week of cardio with weight training or up to 7 hours/week of cardio by itself, is in the right range for most people.

Your specific situation running for 2.5 hours/week plus weight training sounds just fine, so don't worry about it if you are enjoying your program and getting good results.