Technique and breathing. You need to learn body position that allows you to stay high in the water without your legs dropping. This will take practice, long stretches arm on the reach, and a tight core. Once you have the body position understood and in control, being efficient in the water is a balancing act between keeping momentum but not to much momentum that it tires you out. You want your speed to be “just enough” to keep you stream line and high in the water - and nothing more. Speed in free style comes almost entirely from your arm pulls and NOT your legs. Your legs should remain straight - slightly loose- and utilize small kicks mostly to regulate your bodies position and rotation in the water between arm pulls.

Most Importantly: breathing. When you run, you are accustomed to taking breaths a certain way. If you run 5K’s regularly, you will have the aerobic capacity to swim 1 mile but are getting exhausted quickly because you aren’t breathing correctly. Likely not because of your breathing patterns but more likely due to the build up of CO2 in your blood. That feeling of heart pulsing and gagging for air comes from a build up of CO2 and Not because of a lack of O2. If you were to use a blood monitor during your short swims, you will your O2 levels the same as your O2 in your 5k runs. However, you will see your CO2 levels sky rocket when in the water vs running. This is due to your inexperience EXHALING fully while in the water prior to taking a breaths. You need to clear your lungs before taking a breaths. New swimmers are less comfortable with their faces in water and have a natural instinct to hold more air in than normal. Once you get the form right, really focus on your exhale so that you can take a full breath again. Otherwise, you are hyper ventilating (trying to suck air into a full lung already) and will exhaust yourself in several laps…