Showing up to the gym without a plan means you’ll wander the floor deciding what to do, wasting precious minutes in the process. What's more, you may end up doing a workout that won’t help you achieve your end goal.
Perkins recommends asking yourself a couple questions before you even leave your desk: Am I just looking to burn the most calories in the least amount of time? Or do I have a larger goal I need to work toward? Here's an example: “If a client tells me her goal is to put on lean muscle mass, but aside from seeing me once a week, she's mostly spinning on her lunch breaks because it’s convenient, she will have trouble achieving that goal,” explains Perkins. “An overall strength training workout is something you could definitely fit into a lunch period, but you may need to forgo some spin classes in order to meet your goal.”
Perkins advises spending some time on Sunday pre-planning your workouts for the week. Mark the days on your calendar you expect to fit in exercise over lunch, as well as any days you don't plan to work out. You should also write some notes (in a planner, on Post-Its, in your iCal, wherever) about the type of workout you plan to do—and why.
Just remember, as convenient as lunch workouts may be, there are some goals that may not be achievable solely in these shorter sessions (marathon training, anyone?). Coordinate wisely and be flexible about carving out time before and after work when necessary.