Editor’s note: This is a guest post by Jon Finkel.

It was the immortal Rodney Dangerfield in Back to School who said, “You gotta look out for number one… But don’t step in number two!”

Yes, Dangerfield’s character in the movie was known for armpit farting and doing the esteemed Triple Lindy dive, but he was also a self-made millionaire and the CEO of a major company. He even cared about his son graduating college so much that he actually moved into a dorm on campus (a souped-up dorm, of course) and enrolled in school with him. The above quote is the cornerstone of his commencement address at the fictional Grand Lakes University. In the speech, he was talking about going out into the real world, but the same rule applies to men as they enter fatherhood.

After being your own “number one” your whole life, once you have a baby, you get knocked down a peg. Looking out for number one now means looking out for your kids. Not stepping in number two is a combination of the following: stress, 3am feedings, 6am feedings, no sleep, no time to exercise, fast food, caffeine, candy, sugar, skipped breakfasts, rushed lunches, high fat dinners, energy loss, weight gain and, eventually, a chubbier, slower, exhausted you. Most men step right into a steaming pile of the above immediately after becoming a father. The result: a man you barely recognize in the mirror.

At a time when you should be at your absolute physical peak, you’re spent. Why does this happen? Because the one thing we took for granted throughout most of our lives as single men, or as part of a married couple with no children, is long gone: free time. Once you have a baby, your time is no longer free. Every second costs something. If your baby is colicky and it takes you an extra half hour to get her to sleep after her 4am feeding, that costs you 30 minutes of sleep. If you unexpectedly run out of formula in the morning and you have to race out to get some before work, that costs you 20 minutes at the office. If you’re giving your kid a bath before bed and she poops in the tub, the cleaning time costs you 10 minutes you could have had alone with your wife. Every second you spend looking out for the new number one, no matter how wonderful and amazing and fun that can be, costs you a second that you didn’t have to worry about when you were number one.

Eventually, all those seconds add up to a time-stealing conspiracy that forces you to start eliminating things you once looked forward to: reading for pleasure, re-watching The Godfather on HBO on a Tuesday night just because it’s on, spending Sunday watching football. Whatever it is you did in your spare time you no longer do because you don’t have time to spare. Unfortunately, the first thing that usually gets cut isn’t one of the lazy activities I mentioned above. No, the first thing to get cut is the thing you actually like the least (but will help you the most): working out.

Maybe you lift weights, maybe you play hoops, maybe you run or play racquetball or swim or bike. Maybe you simply bang out 30 minutes on the treadmill three days a week. Whatever you normally do, the one thing I can assure you, at least for a while after you have a kid, is that you will no longer “normally” do anything. Having a child redefines what normal will be. And while there are countless magazines and blogs and books for women about pregnancy and exercise and post-pregnancy exercise, there is a staggeringly small number of outlets for men to learn how to take care of themselves after their child is born. Somehow, society has made it acceptable, cute even, for a man to gain weight along with his pregnant wife. Some men even go pound for pound with their significant other. Unfortunately, men don’t have access to the one-day miracle weight loss plan that women utilize called giving birth.

One minute, a woman is bursting at the seams with child, the next minute; she’s 15 pounds lighter. For men, what was once affectionately called “sympathy weight” is now referred to as “being fat.” Once you’re fat for a certain amount of time, you have officially “let yourself go.” And once you’ve “let yourself go,” it’s not easy to get yourself back.

In order to avoid this all-too-common fate of fatherhood, you need to come up with a strategy to help you bridge the gap between your, “I’m in great shape and about to have a kid,” days and your, “My kid sleeps all night and I sort of have my life back,” days. Ideally, this will help you avoid the, “I haven’t worked out or slept in three months, I eat like crap, and I’ve gained 15 pounds,” days.

For all the dads out there who think they don’t have the time to exercise, if you stop and think about it, you do have the time, it’s just hiding in plain sight. It only takes a minute or two here and there to crank out some push-ups, sit-ups, body squats, or something else just to get the blood moving. Want some examples? No problem. Here are five times in your day where you’re burning a minute or two and don’t even realize it:

While the water is getting warm in the shower: It usually takes about a half-minute for the water in your shower to get to the temperature that you’d like. Turn the water on and instead of just standing there looking at yourself in the mirror, drop and do a set of push-ups.

At night, while the water is getting warm in the tub for the baby: One of the first things you’ll need to understand as a new dad is that you don’t have to hold the baby every second to keep it from crying. It’s okay if it cries for a while. This means that you don’t have to hold the baby on your knee if you’re readying a bath. Put her down for a few minutes of all-important “tummy time” (this usually makes them cry anyway, so you might as well have her work on her upper-body strength while she’s bawling), get in a quick set of sit-ups or leg lifts, then pick her up when the water’s ready.

While you’re warming a bottle: Whether it’s first thing in the morning or late at night, it usually takes a few minutes to get your baby’s bottle to the ideal temperature. If you’re using formula or your wife is pumping and freezing the milk, you’ll have a few minutes while the milk soaks in hot water and warms up. That’s plenty of time for a quick set of body squats and for you to check your fantasy football standings on your iPhone.

While your computer is turning on: I found this one ideal for the office. It takes, on average, 2-3 minutes for your office computer to boot up and load all of your company’s firewalls and stuff. When you first get into work, turn your computer on, shut your door, bang out a quick set of push-ups, open your door and start your day. If you’re in a cubicle, find an empty conference room and do it. Yeah, it’ll be weird if you get caught, but so what?

Anytime you microwave something: This mainly goes for when you’re home, but if you’re heating up or defrosting something, chances are you’re just going to pace the kitchen, stare at the timer, or putter around the house for a few minutes. Make those minutes count by doing a set of push-ups or squats.

And that’s just the tip of the iceberg. When you stop and think about it, you can turn dozens of everyday activities, chores, and even burping your kid into a calorie burner and muscle builder. Looking out for number two just got a whole lot easier.

How did you stay in shape as a new dad? Share your tips with us in the comments!

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For tons of manly advice about how to stay in peak form when you become a father, Jon Finkel’s book, The Dadvantage: Stay in Shape on No Sleep, with No Time and No Equipment, is available today. For more information on Jon, The Dadvantage, or his previous work, visit www.jonfinkel.com. Follow Jon on Twitter: @3dollarscholar. Jon Finkel has written for GQ, Men’s Health, Men’s Fitness, ComedyCentral.com, and the New York Times, among others.