I recently started training for a marathon this spring. It'll be my third go at the 26.2-mile distance, so I have all sorts of goals for it. One of my goals, you might be surprised to hear, is not to gain weight during training. Huh? Gain weight? Yep. Let me explain.

While running burns all sorts of calories, some people (myself included) will actually see the number on the scale stay the same—or even increase. Despite their best efforts, some runners struggle to lose weight. If you've taken up running or you're working to run farther or faster, keep these three mistakes in mind so you don't pack on the pounds.

Eating more calories than you burn

One common reason why some runners don't lose weight is because they overestimate the amount of calories they can consume during training. And it's easy to do, especially if you're training for a long-distance race like a half or full marathon. Running burns about 100 calories per mile, and with all the running you do, you probably think it's okay to indulge in your favorite foods when the "post-run munchies" hit, but, as we know, eating calories above and beyond what we burn will lead to weight gain. Of course, it's normal to feel hungry when training, but stocking your kitchen with healthier foods and thinking twice about what you put into your body will ultimately help you keep off the pounds.

RELATED: 11 Ways to Stop Overeating After Your Workouts

Doing the same workout day after day

Getting into a running habit is good for your health, but if you do the same running workout (e.g. the 3-mile loop around your neighborhood that you love so much), day after day, your body might get a little too used to it. Your muscles continuously adapt to the demands that you place upon them, so if you're running the same route over and over again, you might not see the scale move, or worse: you might see it move in the wrong direction. Instead of doing the same steady-state run, mix up your workouts with sprint intervals, hills, and vary the surfaces you run on (pavement, dirt, grass, sand) to keep your muscles guessing and growing stronger.

RELATED: 15 Running Tips You Need to Know

Focusing too much on the scale

If the scale isn't budging, try not to focus on the number too much. Running works the strongest muscles in the legs and since the quadriceps and hamstrings are integral to running, you're toning your lower body while burning fat. At the same time, you're building muscle in your legs, which weighs more than fat tissue, so what the scale says isn't a great reflection of overall fat loss. On the flip side, muscle is more dense and takes up less space, so your clothing probably fits better. Your weight might not have decreased, but your other body measurements have likely changed, such as your waist circumference. In the end, you really just want your "quads of steel" to help you cross the finish line, right?

RELATED: 7 Tips for Running Your First Race

Read Tina’s daily food and fitness blog, Carrots ‘N’ Cake.