How to Turn Your Training Data Into a Race-Day Fueling Plan

You have months of data sitting on your wrist. Heart rate files. Pace splits. Calorie estimates. Garmin Body Battery scores. Training load graphs. Long run after long run, all logged and synced.

And yet, when someone asks you "what's your race-day nutrition plan?", you hesitate.

That gap between having data and knowing what to eat at mile 18 is where most race-day nutrition plans fall apart. Not because athletes don't train hard enough, but because nobody taught them how to read their own numbers and turn them into a fueling strategy.

Here's how to do it.

Step 1: Find Your Actual Effort Level, Not Your Pace

Pace is what you ran. Heart rate is what it cost you.

A 9:00/mile long run in July heat at 165 bpm is a completely different metabolic event than a 9:00/mile long run in October at 148 bpm. The first one burned through glycogen faster, produced more sweat, and demanded more electrolytes. The second one was aerobically easier — your body relied more on fat oxidation and less on carbohydrates.

Pull up your last 4–6 long runs and look at the average heart rate for each one. If you ran roughly the same pace but your heart rate dropped over the training block, your aerobic fitness improved — and your fueling needs per hour at that pace actually decreased.

What this means for race day: Your carb-per-hour target should be based on the effort level (heart rate zone) you plan to race at, not the pace you hope to hit. If you're racing in Zone 3 for 3.5 hours, you need more carbs per hour than someone racing in Zone 2 for 4.5 hours at the same pace — even though the slower runner is out there longer.

Step 2: Use Heart Rate Drift to Spot Fueling Failures

Heart rate drift is when your heart rate gradually climbs during a run even though your pace stays the same. Some drift is normal — it's called cardiac drift and it happens due to rising core temperature, dehydration, and glycogen depletion.

But excessive drift — more than 10–15 beats over the course of a long run — is a signal that something broke down. And in most cases, it was fueling.

Go back through your long run files and look for the ones where your heart rate spiked in the final third. Now ask: what did you eat and drink during that run? If you under-fueled, your body ran out of readily available carbohydrate, shifted to fat oxidation (which requires more oxygen), and your heart rate climbed to compensate.

What this means for race day: Those long runs where drift was minimal? That's what a well-fueled effort looks like. Whatever you ate and drank during those sessions is your starting template. The runs where drift was excessive? That tells you exactly where your fueling broke down — and you now know not to repeat that pattern.

Step 3: Calculate Your Personal Carb Ceiling From Training

There's a reason the advice ranges from "30–60g of carbs per hour" to "60–90g" to "up to 120g." The right number depends on your body weight, your effort intensity, and — critically — what your gut has been trained to handle.

Here's the practical way to find yours:

1. Start with body weight. A rough starting point is 0.7–1.0g of carbohydrate per kilogram of body weight per hour for marathon-intensity effort.
2. Adjust for intensity. If you're racing in upper Zone 3 or Zone 4 (relative to your max), move toward the higher end. If you're in lower Zone 2 to Zone 3, you can stay at the lower end because your body is oxidising more fat.
3. Validate against your training. Look at your successful long runs — the ones where you felt strong all the way through, where heart rate drift was minimal, and where you didn't bonk or hit a wall. How much did you actually consume? That's your proven ceiling.

Don't chase the 90g/hour number because you read about elite runners doing it. If your gut handled 50g/hour well during every long run and you felt strong, that's your number. Race day is not the time to experiment upward.

Step 4: Read Your Sweat Patterns Across the Training Block

Your watch estimates calorie burn, and some Garmin models now estimate sweat loss. But even without that feature, you can calculate your sweat rate from training data you already have.

Weigh yourself before and after a long run (without clothes, before rehydrating). Every kilogram lost is roughly one litre of sweat. Factor in whatever you drank during the run and you have your hourly sweat rate for those conditions.

The key insight: your sweat rate changes across the season. An athlete who sweated 1.5 litres per hour in August might sweat only 0.9 litres per hour at the same effort in October. If you built your hydration plan on summer data and your fall race is 15 degrees cooler, you'll likely overhydrate — which brings its own risks.

Check your sweat rate from your most recent long runs in conditions closest to what you expect on race day. That's the number your hydration plan should use.

Step 5: Map Your Calorie Estimates to Real Products

Your watch says you burned 2,800 calories on your 20-miler. That number is almost certainly wrong — Garmin and Apple Watch calorie estimates can be off by 15–30%. But the relative pattern is useful.

Look at calorie burn per hour across your long runs. If you consistently burn 700–800 calories per hour at race pace, and you know that roughly 60% of that comes from carbohydrate at your intensity (based on your heart rate zone), then you need approximately 420–480 calories from carbs per hour. That's 105–120g of carbohydrate.

Now translate that into actual products:

• A typical gel is 22–25g of carbs
• A 500ml bottle of Skratch is 20g of carbs
• A Maurten 320 mix is 80g of carbs per 500ml

If your target is 60g/hour and you're carrying gels plus a drink mix, that might be one gel every 30 minutes plus sipping your drink mix. If your target is 80g/hour, you might need a higher-concentration drink mix plus a gel every 25 minutes.

What this means for race day: Don't just know the number — know the exact products, exact quantities, and exact timing that add up to your target. Write it on your arm or tape it to your water bottle.

Step 6: Build the Minute-by-Minute Plan

Take everything from the steps above and construct a simple race-day timeline:

• Miles 1–3: Settle in. Sip water or electrolyte drink at the first aid station. No gels yet — your pre-race breakfast is still fuelling you.
• Mile 4–5: First gel. Your body is warm, your gut is active, and you want to get ahead of glycogen depletion before it starts.
• Every 20–30 minutes after that: Gel or chews on your schedule, not when you feel hungry. By the time you feel the bonk, you're already 30 minutes behind on fueling.
• Every aid station: Water or sports drink, alternating based on your hydration plan. If the race provides a drink mix you haven't trained with, stick to water and your own products.
• Miles 20–24: This is where under-fueled runners fall apart. If you've been consistent since mile 5, you'll have enough glycogen to push through. Consider a caffeinated gel here if you trained with caffeine on long runs.

The Problem With Doing This Manually

Everything above is doable. You can pull heart rate files, calculate sweat rates, estimate carb needs, and build a timeline on a spreadsheet. Many serious athletes do exactly this.

But it takes hours. And it requires you to correctly interpret data that most athletes — even experienced ones — misread. A heart rate spike at mile 16 could be dehydration, under-fueling, pacing error, or heat stress. Knowing which one requires cross-referencing multiple data streams.

Sweatr does this automatically. It pulls your training data from Garmin, Apple Watch, and Strava, calculates your personal sweat rate and carb needs, adjusts for race-day conditions, and produces a fueling plan mapped to real products — with timing reminders on race day.

You spent months building the data. Sweatr turns it into the plan.

What To Do This Week

If your fall marathon is 2–6 weeks away:

1. Do one more sweat test on your next long run. Weigh in, run, weigh out. Calculate your sweat rate for current conditions.
2. Review your last 4 long runs. Note the heart rate drift pattern. Identify which fueling approach produced the least drift and the best final miles.
3. Lock in your products. Choose your gels, drink mix, and electrolytes now. Race week is too late to switch.
4. Write the plan down. Minute by minute, mile by mile. What you'll consume, when, and from where (your own supply vs aid stations).

Or connect your watch to Sweatr and let it build the plan from your data in minutes. Either way — don't show up on race morning without a plan you've tested.