5 surprising facts about getting into shape, according to a professional trainer

Training for a competition to get fit — a triathlon, 10K, mud-and-obstacle run, or something else fun — is an excellent way to stay motivated.

But if it's your first time preparing for that kind of physical challenge, you're likely to learn a few unexpected things along the way.

Business Insider spoke with Dan Arnett, a professional triathlon coach, to see what surprises people encounter while getting into shape. Here are his top five.

1. You'll gain weight.

With daily (or almost-daily) workouts, those extra pounds should just melt off, right?

Not exactly. Arnett says most people, especially men, put on some weight initially. This happens quickly too, within the first two to four weeks.

Your body adds muscle mass at the start, which makes you weigh more, even if you lose extra flab around the waist. If you've got significant weight to lose, it should come off eventually. But don't look for an immediate decrease on the scale — especially if you haven't altered your diet.

Focus on how your clothes fit instead, says Arnett.

2. You do not need a coach.

If you've signed up for a competitive event, especially something like a triathlon, you may think you need a professional coach to get you going.

But Arnett says you don't need to pay a professional if you're just getting started. Instead just create a good training plan — you can usually find one for free online.

If you want to get really serious after finishing your first few races, then you can look into a coach.

3. You still can't eat whatever you want.

It's easy to view the fact that you're burning a few hundred more calories a day as a license to indulge in all the cheeseburgers and ice cream you want.

"People expect to be able to change their diet" Arnett says.

But most of us can't eat whatever we want all the time, even if we are working out. Most people are usually are getting enough calories, so there's no need to suddenly start carb-loading.

Those training for an ultra race like an Ironman may need some additional nourishment, but they still need to eat healthy — and a person training for a first race won't get close to that level of calorie-burning.

So go ahead and enjoy the occasional cheeseburger or ice cream as you ordinarily would — but keep it occasional.

4. Some of the greatest benefits of your training might be related to mental health.

You're likely trying to get in shape because you want to feel better physically, but that's not the only way that a training program can improve your life.

Research shows that exercise can reduce anxiety and depression, improve sleep and cognitive function, and raise self esteem.

"Once they get that workout done, people say they feel so much more productive during the day," says Arnett. What's more, the sleep perks and mental health boost help motivate you to keep up with your workouts — a wonderful cycle, once you get into it.

5. Sometimes, it's better to skip your workout.

You don't want to miss a workout, especially when you are getting ready for a big event. Arnett recommends putting your training schedule into your calendar and treating workouts like work meetings that you never miss.

But — like with your job — there are legitimate reasons that you might need a day off.

If your family needs your attention, they should come first, Arnett says.

You also don't want to hurt yourself, so watch out for overuse injuries. If you've been "ramping up, doing too much, and not listening to your body," Arnett says, you may hit a point where it's better to rest, even if it's not your scheduled off day.

Do your body a favor: Take that break.