Exercise: How Much Is Enough?

If you don't mix up your exercises during a single session, vary them from session to session, Massaro suggests.

"Once your body gets used to doing something, you don't get as many benefits from doing it. So either you have to increase the time or intensity or keep changing up the movements to keep your body guessing," he says.

Among Massaro's favorite quickie exercises are basic jumping jacks and squat thrusts, along with walking -- but with a twist.

"To make it into a challenge, try walking in a zigzag pattern, or even walking backwards. It looks a little weird but it definitely challenges your muscles more," he says.

If you are going to do a short workout, Ryan says, make it as intense as you can to get some cardio benefits.

"You need to put some kind of intensity behind whatever activity you're doing if you really want to continue to gain benefits from these short bursts of activity," he says.

So, if you're walking, speed it up. If you're bending and reaching, challenge yourself to do more repetitions in the same time frame.

Getting Motivated to Exercise

While it might seem as if doing just a little exercise won't require much motivation, experts say that isn't so. Because the sessions are so short, it's easy to put them off, or even blow them off, without guilt.

"If you miss an hour of working out with a personal trainer or an hour at the gym, there's a certain amount of guilt attached that can motivate you not to skip out. But when you can skip 5 minutes, of exercise on your own, it doesn't seem like such a big deal. … So unless you stay motivated, it's easy to get sidetracked away from your goals," says Shina.

To stay focused, Massaro says, keep your eye on the prize: how good you'll feel and how much healthier you will be if you stick with your exercise program.

"Don't think about what you have to do, think about what you will get if you do it. Namely, you'll feel better, you'll look better, your health will be better," he says.

If you still need more motivation, pick an exercise buddy and set up a competition, Shina suggests. "First, each of you buys a gift card to a favorite store. Then, each of you must write down all your fitness activities -- when you did them and for how long. And at the end of the week, compare notes. Whoever did the most for the longest gets to have both gift cards."

Ryan says you can also stay motivated by competing with yourself. "Staring out with short bouts of exercise is a good way to establish the fitness mind-set, but you should continually challenge yourself by making your end goal the ability to work out for 30 minutes at a time, three times a week. That can be a very motivating challenge," he says