What Actually Helps Endurance Athletes Feel Stronger on Long Training Days

Endurance athletes feel stronger on long training days when four things are handled well. They replace the fluid and electrolytes they actually lose, not a generic amount. They avoid overcorrecting with too much fluid or sodium. They fuel adequately for the session’s length. And they recover deliberately afterward. Sweat losses vary widely between athletes and even within a single session, so measuring your own losses is the most reliable way to stop guessing.

Why Long Days Leave You Feeling Depleted

The late-session fade is rarely about fitness alone. As fluid losses build, total blood volume drops and the heart has to beat faster and harder to keep oxygen moving. This is known as cardiovascular drift. For every 1% of body weight lost to fluid, heart rate rises by roughly 3–5 beats per minute at the same workload. Less blood reaching working muscles and the skin also means faster fatigue and less effective cooling, so a steady effort feels harder than it should. The consequences of dehydration are covered in more detail here. 

Carbohydrate matters too. According to the joint position paper from the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine, depleted glycogen stores are linked to reduced work rates, impaired concentration, and a higher perception of effort. That combination is what “depleted” feels like.

1. Know Your Fluid Losses

Sweat rates during exercise range from roughly 10 to 80 fl oz per hour, depending on intensity, duration, fitness, heat acclimatization, and environmental conditions. The spread is just as wide within a single sport. Across outdoor runs of three hours or longer, Nix data shows fluid losses ranging from about 12 to 45 fl oz per hour. Female runners averaged 21.4 fl oz per hour and male runners 30.7 fl oz per hour.

The practical target is to keep total fluid loss under 2% of body weight, the threshold at which aerobic performance and cognitive function can decline, particularly in the heat. For a 160 lb athlete, that is about 3.2 lb, or roughly 51 oz of net deficit. At higher sweat rates, full replacement often exceeds what the gut can absorb, so the goal becomes managing the deficit rather than erasing it.

2. Replace Electrolytes, Not Just Water

Sweat carries sodium, chloride, potassium, magnesium, and calcium. How much you lose does not track neatly with how much you sweat. Heavy sweaters don’t necessarily lose the most electrolytes. Across long training runs, electrolyte losses ranged from about 420 to 2,800 mg per hour, and they varied roughly twice as widely between athletes as fluid losses did.

Sodium makes up roughly 47% of total electrolyte losses on average. That is why Nix reports total electrolyte loss with sodium broken out separately. A plan built around sodium alone addresses less than half of what leaves your body. The Understanding Electrolytes and Breakout of Electrolytes articles explain this breakdown in more detail.

3. More Isn’t Always Better

Overcorrecting can be as costly as falling behind. Drinking more than you lose is the primary cause of exercise-associated hyponatremia, a dangerous dilution of blood sodium. The risk is highest for lower-sweat-rate athletes following advice calibrated to the average.

Sodium can be overdone too. During a 15-hour Backyard Ultra, RunLab host Chris Beavon kept to a fixed sodium replacement plan while his measured sodium losses fell to less than half their early-race rate. The resulting surplus likely contributed to the GI distress that ended his race. His full story is in How Much Sodium Is Too Much.

4. Expect Your Numbers to Change

A single sweat test describes one session in one set of conditions. Across comparable workouts, two HYROX athletes tracked by Nix each saw their heaviest session produce roughly double the fluid loss of their lightest. In one half marathon build, race-day electrolyte losses came in about 41% above the training average, driven by warmer conditions and a faster pace. Losses shift with heat, humidity, intensity, and duration, so a plan should be built from multiple sessions and adjusted in real time.

5. Fuel Adequately, Then Personalize With a Professional

Hydration isn’t the only reason long days feel draining. Carbohydrate intake during extended sessions plays a major role in how strong you feel in the later hours. The joint position paper suggests roughly 30–60 grams of carbohydrate per hour for sessions lasting between 1 - 2.5 hours, and up to about 90 grams per hour for efforts beyond between 2.5 to 3 hours. These are general ranges. The right amount, timing, and type of carbs depend on your intensity, gut tolerance, and training goals.

For a fueling plan built around you, work with a registered dietitian, coach, or sports nutritionist. Sharing your Nix fluid and electrolyte data with them gives them a fuller picture of what your long days demand.

6. Recover With Intention

Most athletes finish long sessions with a fluid deficit, and that deficit carries into the next day’s training. The position paper recommends replacing 125–150% of fluid lost, because sweating and urine losses continue after exercise. The Nix Recovery Calculator uses 150% of fluid and 100% of electrolytes lost, consumed over the two hours after a workout. For example, an athlete who loses 27.6 oz of fluid and 2,242 mg of electrolytes would aim for about ~41.5 oz of fluid and 2,242 mg of electrolytes in that window.

Putting It Together

Start with a simple pre- and post-workout weigh-in to establish a baseline sweat rate or skip the nude weigh-in altogether and use the Nix Hydration Biosensor to measure fluid and electrolyte losses across different conditions and session types. Dig into your Sweat Profile and personalized insights, test your plan in training, and export your data to share with your coach or dietitian.

Frequently Asked Questions

Why do I feel wiped out late in long sessions even when I drink a lot? Fluid is only part of the picture. You may be under-replacing electrolytes, under-fueling, or even drinking more than you lose. Measuring your actual losses shows which one is the key factor.

Is drinking to thirst enough on long training days? Thirst is a useful signal, but it lags behind your actual fluid deficit and tells you nothing about electrolyte losses. High-loss athletes tend to under-drink when relying on thirst alone.

Do I need to replace sodium or all electrolytes? All of them. Sodium accounts for the largest share, but it averages less than half of total electrolyte losses.

How much should I drink after a long training day? Aim for 150% of the fluid you lost, along with the electrolytes, over the two hours following your session.

References

1. Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016;48(3):543–568. Full text via Dietitians of Canada

2. Baker LB, et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes. J Sports Sci. 2019;37(20):2356–2366.

3. Nix Biosensors. Hydration Tracking for Marathon Runners: What the Numbers Actually Look Like