Everything seems cool until you're naked. You look slim with your clothes on, even after a lifetime of eating whatever you want. But the mirror, the doctor, and your girlfriend all know you're skinny-fat: no muscle mass, no definition, and no mojo come beach season. The condition afflicts lightly built men in their late twenties and early thirties, especially burger-loving, beer-guzzling bros who limit their workouts to casual cardio. Sound familiar? Fortunately, you don't need to go all America's Biggest Loser. You just need to tone up, and you can do that in ten weeks, no sweat. Well, a little sweat.

1: Stop Focusing on Your Weight

Start: Looking at a whole new number: your body-fat percentage. A skinny-fat guy starts out at 16 to 25 percent body fat, says trainer Will Torres, who runs Manhattan's Willspace gym. The goal is dropping to between 9 and 12 percent. That's where you can actually see abs, plural. A good electronic scale like the Fitbit Aria ($130) can measure your percentage. For the most accurate results, hop on when you're hydrated, sober, and haven't worked out for eight hours (e.g., midday).

2: Stop Running So Damn Much

Start: Flexing some iron. Skinny-fat guys typically avoid the weight room, fearing they'll get huge. They won't—as Torres says, lifting is the fast track to replacing fat with muscle, which is what skinny-fat guys need. And don't bother with isolation workouts. "What's better: doing biceps curls or doing pull-ups?" asks Torres. "Pull-ups, because they're working your back, your core, and your arms." Dead lifts, burpees, and other hybrid moves (see right) have a similar effect: You'll look cut all over, rather than like a pair of pecs mounted on two legs. Once you're strength training, put your workouts on shuffle. Every week, change the ercises you're doing. And when lifting gets easy, bump up the weight or reps. Even minor tweaks (like swapping a barbell for individual dumbbells) make a difference. Gym rats call this "muscle confusion." Expect to be sore.

3: Stop Eating (and Drinking) Like a College Kid

Start: Respecting your body type. While you may be able to scarf those cheese fries without busting through your jeans, you'll never look better until you start eating the way you know you should. Go for lean proteins, pile on the fresh vegetables, and cut out as much sugar and dairy as you can stand. Two more clutch nutritional moves: First, your new snack for the next ten weeks is a protein-packed handful of almonds. Second, at the bar, you're gonna order tequila on the rocks with a splash of soda and lime. It's stronger (and manlier) than those watered-down light beers, so you get more buzz with about the same calories. Just please don't call it a "skinny margarita," okay?

Twice the Burn, Half the Time

Flatten your spare tire with trainer Will Torres's killer combo move:

Step 1Stand with your feet together, holding a dumbbell in each hand at shoulder height.

Step 2Step back with your left leg and lower your body until your right knee makes a ninety-degree angle. As you lower yourself, raise the weights straight up.

Step 3Step forward and return to the starting position, dumbbells at your shoulders. Do three sets of fifteen reps for each leg.