One workout may not undo a sedentary lifestyle.
Working out really matters for your health and longevity, but more research these days is telling us that simply exercising—whether 10, 30, or even 60+ minutes a day—may not be enough to offset the effects of an otherwise sedentary lifestyle. Just because you exercise doesn't mean it's OK to be a couch potato the other 23 hours of the day. Sitting, driving, working from a computer, sleeping—all of these "inactivities" make up the bulk of many people's days, and the longer you sit still, the worse the effects can be on your health. I talked a little about "activity" vs. exercise above. This is where those extra non-workout activities DO matter. They may not be true workouts, but they do have benefits. More movement is good—and that is how you achieve the benefits of an active lifestyle.
You're not burning as many calories as you think.
"Burn up to 800 calories an hour!" How often do you see phrases like that advertised on workout DVDs, group classes, and other fitness products? The truth is, most of these numbers are seriously inflated, and the average person won't burn a fraction of that claim. This is the case for treadmills, stationary bikes and other cardio machines, too. Those "calorie burn" screens can be off by 30% or more. SparkPeople tries to be a little more conservative with the numbers we use on our Fitness Tracker, but just remember that calculators/trackers are estimates. When it comes to weight loss, you're better off with a conservative approach to calorie burn. Assume you're actually burning fewer calories than a tracker or machine says you are. A better way to gauge what you're really burning is by wearing your own heart rate monitor. While a general fitness tracker would tell me that an hour of Spinning burned some 600+ calories, my HRM (using my gender, weight, and actual heart rate during the workout) showed closer to 400. That's a big difference that could really affect one's weight loss.
It won't allow you to eat whatever you want.
A walk around the block doesn't earn you a brownie. That yoga class doesn’t mean it's OK to indulge in an ice cream sundae this weekend. How often do you "reward" yourself for working out by undoing most of your efforts with one or more dietary splurges? Remember, exercise really doesn't burn as many calories as people assume it does, so a single workout—even a rigorous one—won't come close to offsetting just ONE big splurge. Yet I know many people who justify their food choices by saying "I worked out today." If weight loss is your goal, you have to keep these splurges in check; otherwise, you'll be fighting a losing battle and never really get ahead in the calorie equation.