"So ramp up slowly over two to three weeks," says Breakthrough founder Jonathan Roche. "Guys, in particular, will go all-out and waste themselves." Properly done, a high-intensity work interval should be followed by a low-intensity rest interval that allows your heart rate to recover, or come down to a level where you're breathing comfortably. Generally, the more intense the work interval, the longer the rest interval.
From a psychological perspective, intervals are tricky.
"They're more fun because we like to be challenged to do better, but they are more gut-wrenching and grueling," says Ron Jones, an Atlanta- and L.A.-based corporate wellness coach. "Although we know that lukewarm goals don't work very well, too-hard ones can frighten you away."
Like Roche, he advises taking it easy at first with slower, shorter efforts. "I've had people do five-second intervals," he says. "Then slowly -- slowly -- build on that success. Remember that it takes three weeks to psychologically form a new habit, and six months to change a behavior. Even seeing the physical changes that come with interval training may not be enough to let you stick with it. You have to feel good about what you are doing."
Cuellar, now 20 pounds lighter than six months before, thinks she's gotten to that point. "My husband has offered to buy me a whole new wardrobe," she says. "But I told him to wait until I get down to a size 6."