You've decided you need to get in shape. You could choose to go to the gym, and sweat behind four walls with a bunch of other guys, grunting and throwing weights around in the stale air.

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Or you could head outside, strap on a pack and hike yourself into shape along some of the best trails in the country.

Seems like a no-brainer. And it was to Steve Silberberg, who quit his high-paying computer programming job 10 years ago to found the “adventure weight loss company" Fitpacking. Since then he's been taking people who want to increase fitness, lose fat, and build muscle, on backpacking trips around the US, South America and Israel.

Fitpackers only eat what they can carry

“It's the perfect way to kickstart a fitness regimen and diet," Bree Gotsdiner, Director of Media Relations for Fitpacking, told me. “And to learn portion control." Portion control is important, since hikes run between one to two weeks, depending on the trail, and you only eat what you can carry with you.

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The guided hikes are rated on a scale of 1 to 4 stars — 1 star is the easiest, such as a hike through Olympic National Park in Washington, with 4 being the most challenging, a 2-week hike through Yosemite National Park, for example. That way, even experienced and fit hikers can get a great outdoor workout. (And if you're faster than the rest of the group, Bree, Steve, or one of the other guides, will lead you through some exercise like pushup contests to keep your heart pumping.)

On one of the more challenging legs of a Fitpacking hike

Never been hiking or camping? Don't worry, once you've signed up to fitpack, you'll get a list of gear and food to bring. And if you're a true novice, Steve and his team give you a few tips to get you trail ready: park far from the grocery store and walk, wear hiking boots around town for a few weeks prior to break them in, wear a backpack to get used to it, take the stairs to prep for hills, and other helpful hints.

When hikers arrive, there's a shakedown to make sure you've got the right gear and food. If you've packed cotton shirts (which will make you miserable on the trail), for example, they'll shuttle you into town to buy the right stuff. And they'll help you pare down your gear to the bare minimum, nothing unnecessary that'll weigh you down.

Guides make sure you have only what you need in your packs

Fitpacking trips average between $1,000 and $2,000 depending on location and duration, and all include hotel stay before and after the hike, all food on the trail, and ground transportation to and from the trailhead. They average about 24 trips per year, and are adding locations all the time as people suggest new trails to hike.

Looks like there are no more “I hate the gym" excuses. It's time to get off the couch, strap on your boots and make nature your gym.

It's not just a casual walk in the woods...