The Swimrun Workout You’ve Never Heard of

By Jennifer Ward

In April, Annie Stahl, 29, and Will Mondet, 37, both based out of San Diego, will compete at the ÖTILLÖ Catalina in the World Series distance. It will be Annie’s first swimrun race and Will’s second, after competing at ÖTILLÖ Austin last year. 

Will says after Austin he was hooked instantly: “I’ve been doing triathlons for over a decade, and my favorite part was always leaving the water and running wet to the transition. Now I get to do that for hours. You can’t get more amphibious than a swimrun.”

They’re swimrun partners, and they’re also fitness instructors with Deep End Fitness, a pool-based, functional workout community that’s helping make all kinds of people–from endurance athletes to surfers to everyday people–bulletproof and confident in the water. 

Sound like a natural fit with ÖTILLÖ? We thought so, too. Read on to learn more about this unique phenomenon.

The Origins of Deep End Fitness

Deep End Fitness was co-founded by Don Tran and Prime Hall, both former Marine Raiders with extensive military experience. Both have trained thousands of Marines in water confidence, and the two put their combined expertise and passion together to create a program that optimizes human performance through breath control, underwater movement, and stress adaptation.

The program was inspired by the Marine Combat Instructor Water Survival Course, a rigorous military training regimen that teaches recruits to remain calm and functional in high-stress aquatic environments. Deep End Fitness takes these principles and applies them to a broader audience, making the benefits accessible to anyone willing to step outside their comfort zone.

“Really it’s a form of meditation, but just a more active one,” says Mondet. “It’s about using your breath and the control of your breath to calm the mind and vice versa, calming the mind to allow you to go a little longer underwater–in an environment that is normally uncomfortable for humans.”

Deep End Fitness Basics

While it might look like an underwater aquacise class or some kind of strange hybrid swim/kettlebell class, DEF devotees describe it first and foremost as a stress management tool. Think meditation and breathwork over hauling heavy weights and sweating. 

DEF training is built on three key elements enforced in every class:

1. Breathwork – Every session begins with focused breathing exercises, designed to enhance CO2 tolerance, increase lung capacity, and regulate stress responses. Learning to tolerate higher levels of CO2 can improve endurance, stress resilience, and overall fitness. (More on that below.)
2. Mindset – Much like meditation, Deep End Fitness requires class members to control their internal dialogue. When submerged and holding their breath, participants must quiet their mind and override the instinct to surface. This skill translates directly to real-world stress management.
3. Movement – The workouts incorporate a variety of pool-based movements that challenge both physical fitness and breath control. These exercises can include underwater crossover swims (swimming across the pool in a single breath), gutter-ups (pulling oneself out of the pool similar to a muscle-up), and dumbbell walks (carrying weights across the pool floor to maintain neutral buoyancy). “We have a whole library of different movements,” Will says. 

class structure

Each DEF session, running from 60 to 90 minutes, follows a consistent format designed to fit every participant’s level, from beginner to experienced.

1. Circle of Trust – Every session begins with a check-in where participants share their name and water background. This builds community and trust within the group.
2. Land-Based Warm-Up – A 15-minute warm-up includes breathwork, stretching, and breath-hold drills designed to simulate the upcoming water workout.
3. Water-Based Training – The core of the workout consists of various WOTDs (Workout of the Day). The goal is for participants to push themselves at their own pace while gradually improving their CO2 tolerance and water confidence. 
4. Cool-Down and Reflection – Participants end with guided breathing exercises and a debrief, reflecting on the mental and physical challenges of the session.

CO2 Tolerance and Why it Matters

One of the fundamental training principles of Deep End Fitness is improving CO2 tolerance. But what is this, and why is it important for endurance athletes? 

“What makes us have the urge to breathe is not a lack of oxygen, it’s a rise of CO2,” Annie says. “The more we hold our breath, the longer that CO2 has a chance to rise. Normally when we’re not paying attention, what sends our mind the signal to breathe is that rise in CO2 in our blood.”

Annie adds that as we build CO2 tolerance, we also build our ability to manage stress. As CO2 starts to rise, we feel a sense of discomfort. As we’re able to tolerate more CO2, we can tolerate a higher level of discomfort: “That translates to all aspects of endurance sports,” she adds. “It can also translate to the mental side of things. If you’re feeling a sense of discomfort in the water and you can relax in your mind and tell yourself, ‘I’m OK,’ then you can push yourself.”

To summarize, higher CO2 tolerance translates to:

• Improved blood flow and oxygen delivery – By increasing CO2 tolerance, athletes train their bodies to deliver oxygen more efficiently.
• Enhanced aerobic fitness – The ability to function with lower oxygen levels means greater endurance across sports like swimming, running, and triathlons.
• Stronger stress response control – Learning to stay calm under the discomfort of breath-holding can help individuals manage stress in everyday life, from work challenges to potentially high-pressure or high-stress athletic competitions like swimrun.

Deep End Fitness and ÖTILLÖ

“The people that go to Deep End want a challenge—they want to do hard things,” Will says. “It’s not normal for someone to want to jump into the deep end of the pool and hold on to weights and suffer. It’s the same for swimrun. Run with wet shoes? That sounds horrible, but that’s what draws people to it.”

“Every one of the swimrun people I know, they’ve got a couple screws loose, right? It’s the same with Deep End.”

One of the reasons we were so interested in Deep End Fitness is this natural sisterhood with the sport of swimrun. In fact, 7 DEF coaches from Southern California will compete at ÖTILLÖ Catalina next month! 

Will notes that the mental resilience required for Deep End Fitness translates seamlessly to swimrun: “Being able to calm that voice in your head that may be screaming at you, that’s huge for swimruns,” he adds. 

Annie, a longtime triathlete, says doing Deep End has improved her endurance racing tenfold: “Being able to hold your breath underwater, you have to be able to change your narrative in order to relax. If you’re under water thinking “this sucks, this is hard,” you’re going to come straight back up for air. But, if you’re under water thinking, “OK, you can do this, try to relax, sing a song, distract yourself in some positive way,” those are strategies you can come back to again and again.” 

Why Deep End Fitness Works

Unlike traditional strength training or endurance sports, Deep End Fitness offers a unique blend of mental and physical conditioning without the load of other types of training. You’ll leave feeling tired, but rarely sore, making it a low-stress addition to the sometimes grueling swimrun training regimen. Here’s a recap of the benefits:

• Faster recovery – Water-based training reduces impact on the body, making it ideal for active recovery days.
• Enhanced mental toughness – Learning to stay calm while holding your breath under stress builds resilience that carries over into all aspects of life.
• Improved lung capacity and VO2 max – Training in an oxygen-deprived state forces the body to become more efficient at oxygen delivery, leading to better overall endurance.

A Growing Movement

What started as a water survival training program developed by elite military veterans has evolved into a nationwide movement, drawing in athletes, military personnel, and everyday people looking to build resilience, strength, and mental fortitude.

Deep End Fitness has expanded across the United States, with strong communities in cities like San Diego, Austin, and Miami. Recently, they expanded internationally, launching their first chapter in London. 

If you’re looking to spice up your swimrun training, build mental resilience, improve breath control, and push your physical boundaries, Deep End might be worth a look. Check out their website and app to find a location near you. If you’re not located close to one of their class offerings, try a one-week trial of their online program.  

And if you see Annie and Will at ÖTILLÖ Catalina, they’ll be easy to spot: They’ll be the ones looking calm and collected all day long.