The Missing Miles: Why Fueling Determines Your Marathon Success

As the marathon season approaches, runners are focused on long runs, pace work, and taper strategy. Training plans are dialed in. Mileage is logged. Workouts are checked off.

But there’s one piece of preparation that doesn’t show up on your GPS watch — and it may be the most important of all:

Fueling.

As a Registered Dietitian specializing in sports nutrition, I often remind endurance athletes that performance is not built on miles alone. It’s built on how well you support those miles with consistent, intentional nutrition.

Training Is Stress. Fueling Is Adaptation.

Every run places stress on your body. That stress is what stimulates progress — stronger muscles, improved cardiovascular efficiency, better endurance. But adaptation doesn’t happen during the run. It happens after. Without adequate energy (ie. calories) and nutrients, the body struggles to repair, replenish, and rebuild. Over time, this can show up as persistent fatigue, plateaued performance, increased injury risk, disrupted sleep, or difficulty hitting paces that once felt manageable.

Training creates the opportunity for improvement. Fueling allows it to happen.

Endurance Running Is Energy-Intensive

Preparing for a half or full marathon dramatically increases your body’s daily energy demands. Even when runners feel “used to” the workload, the physiological cost remains high.

Endurance running relies on aerobic energy production, meaning your body requires oxygen to burn carbohydrate and fat for fuel. When intake doesn’t match output, the body compensates. Hormones shift. Recovery slows. Energy availability drops. What feels like a motivation issue or a tough training block is often an under-fueling issue in disguise. Because of the increased need for energy, long distance runners require a balanced diet rich in carbohydrate, protein, and fat. Timing and recommended serving sizes vary (so always check with a Registered Dietitian who specializes in sports nutrition for specific recommendations).

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Runners are disciplined with their training plans — but nutrition deserves the same level of structure and attention.

Race Day Is a Reflection of Habits

One of the biggest misconceptions I see is that race day fueling is a separate strategy. However, your body performs best on race day when it has been consistently fueled throughout the entire training cycle. Digestive tolerance, energy stability, and sustained endurance are all trainable — just like pace and cadence. Also, it is just as important to practice with specific fuel sources while you are training, rather than waiting until race day to try out a new product.

You wouldn’t wait until race week to attempt your longest run. Fueling works the same way.

The Taper Isn’t a Time to Pull Back on Nutrition

When mileage decreases, it’s common for athletes to unintentionally scale back food intake too aggressively. But the taper phase is a period of repair, restoration, and preparation. Your body is consolidating months of work. Supporting that process nutritionally is what allows you to arrive at the start line feeling strong rather than flat.

Reducing training volume doesn’t eliminate your body’s need for quality fuel.

Strong at the Start. Strong at the Finish.

If you’ve ever felt powerful through the early miles but completely depleted in the later stages of a race, fueling (or lack of) may have played a role. Late-race fatigue isn’t always about mental toughness. Often, it’s about energy availability.

When athletes prioritize consistent, adequate fueling throughout training, they’re more likely to experience:

Steadier energy levels
More productive workouts
Improved recovery between sessions
Greater resilience during peak mileage
A stronger finish on race day

The Takeaway

As you prepare for your next race— whether you’re tackling 13.1 or 26.2 or anything in between and longer— remember this:

Your performance isn’t determined by training alone. It’s determined by how well you support that training. Because the strongest race-day performances aren’t built in a single long run.

They’re built in the daily decision to fuel like your miles depend on it — because they do.

-Lauren Barkan, MS, RD

Train Hard. Fuel Smart. Race Strong.