The secret to getting through an especially difficult workout? It doesn't matter how strong your muscles get, half the battle is developing a kind of mental toughness to keep going. As it turns out, if you want to survive in the fitness business, the same thing is true.

Speaking at a panel event hosted by American Express' U.S. Small Merchants Group in New York City on Tuesday, the entrepreneurs behind ClassPass, Warrior Fitness Bootcamp, and modelFIT weighed in on their biggest startup challenges, from honing the best marketing techniques to securing funding to the importance of nurturing strong mentor relationships. Sure, the panel organizers may have served up "beauty water" infused with rose petals, but competing in the health and wellness industry is not for the delicate of heart.

Here are some of their top tips for staying--and thriving--in the business:

Don't be afraid to fail.

Payal Kadakia is the co-founder of ClassPass, a subscription service for fitness classes in New York City, San Francisco, and Los Angeles. The company launched in June of 2013, and today it's valued at more than $200 million. To date, the company has raised $54 million in VC funding from firms such as General Catalyst.

Looking back, Kadakia says accepting failure was a key moment for her: "It's okay when something doesn't work out because it gives you another data point for what could work," she said. When her initial business model failed--the company wanted users to pay per class--Kadakia and co-founder Mary Biggins decided to embrace the monthly subscription model. That lead to a rebranding of the company (ClassPass) to become what it is today.

Develop a strong mentor relationship.

All three panelists spoke of the value of having a good mentor. Vanessa Packer, for instance, looks to her father for advice and support, since he too is an entrepreneur. Packer is the co-founder of modelFIT, a company offering nutrition and fitness classes and one-on-one personal training sessions out of its SoHo studio. 

For Kadakia, similarly, having a few key advisers to lean on was instrumental in the early stages of ClassPass, because they helped her to trust her instincts: "People saw more magic in me than I saw," she said, helping her to become "the best possible version of myself." 

Stay true to your own, unique business model.

It used to be that the more heavy machinery and blaring music you had in your gym, the better you could compete in the fitness industry. Of course, now smaller merchants are finding big opportunities in offering city dwellers alternative workouts. Don't feel like running on a treadmill? Try Bikram hot yoga, a Miami-style dance cardio workout à la 305 Fitness, or a myriad of increasingly popular fitness "bootcamps," which borrow from military training techniques.

What sets these companies apart is also what keeps them thriving: At Warrior Fitness Bootcamp, for instance--which offers classes that include partner work, drills, and dumbbells--all hires go through an intensive vetting process, according to co-founder Alex Fell. And most importantly: They're all required to have served in the military at some point.

At modelFIT, by contrast, Packer hires fitness enthusiasts from all different backgrounds. They, too, go through an extensive training process, which she described as "like getting a masters in applied functional science." Packer also believes in letting those instructors maintain their own, unique zest for fitness. Whether it's dance, boxing, or Jujitsu, that expertise should be baked into the modelFIT practice.

Ultimately, all panelists agreed that it's important to have passion for what you're doing. As Kadakia put it: "If you love what you do, it's not work."