Your Watch Logged 50 Runs This Training Cycle — Here's the Race-Day Plan Hiding in That Data

You've spent the last 12 to 16 weeks training. Your Garmin, Apple Watch, or Strava account has logged every run — the long slow ones, the tempo sessions, the intervals that made you question your life choices.

That data is sitting in your watch app right now. Most athletes open it, glance at the weekly mileage chart, and move on.

What they don't realise is that buried in those 50-plus activities is everything they need to build a race-day nutrition plan that's personalised to their body, their pace, and the conditions they'll actually race in.

Not a generic plan from a blog post. Not a calculator that asks for your weight and spits out a number. A plan built from what your body actually did across hundreds of kilometres of training.

Here's how to find it.

The three numbers that matter

Your watch tracks dozens of metrics. Training load, VO2 max estimates, recovery time, stress score, Body Battery, sleep stages, respiration rate. Most of them are interesting. Three of them are essential for building a race-day nutrition plan.

Number 1: Your average heart rate at race pace.

This tells you how hard your body is actually working at the pace you plan to race. Heart rate determines energy expenditure more reliably than pace alone, because pace ignores terrain, wind, and your fatigue state.

Go into your watch app and pull up your most recent tempo or race-pace runs from the last four weeks. Look at the average heart rate during the steady-state portion — not the warm-up, not the cool-down. That number represents your cardiovascular cost at race effort.

Why it matters for fueling: a higher heart rate at race pace means a higher calorie burn per hour, which means a higher carbohydrate requirement. An athlete running a marathon at an average of 155 bpm needs a different fueling plan than one running at 140 bpm, even if they're covering the same distance in the same time.

Number 2: Your estimated sweat loss in race-similar conditions.

If you own a Garmin, check the "Hydration" widget or the sweat loss estimate on your recent long runs. If you have an Apple Watch, look at the calorie burn and environmental data from outdoor runs in conditions close to your race forecast.

The gold standard is weighing yourself before and after a run (nude, towelled dry, accounting for fluid consumed). But your watch data gives you useful proxies: longer runs in similar temperatures, with your heart rate and duration recorded, create a pattern you can use.

Look at your three longest training runs. What was the temperature? How long were you out? If your Garmin estimated sweat loss, average those numbers. If not, use the weigh-in method on your next shakeout run in conditions that approximate race day.

Why it matters: your sweat rate directly determines how much fluid you need per hour. Someone losing 1.2 litres per hour at race effort needs a completely different plan from someone losing 0.6 litres. Generic advice to "drink at every aid station" ignores this by a factor of two.

Number 3: Your calorie burn per hour at race effort.

Pull up your target race-pace runs and look at the calorie burn per hour, not per session. Divide total calories by duration in hours. Do this for at least three runs to find your average.

Yes, watch calorie estimates are imperfect. Garmin's optical heart rate can overestimate by 10-15% on some wrist positions. But the trend across multiple runs is more useful than the absolute number. You're looking for a consistent range, not a precise figure.

Why it matters: your calorie burn per hour sets your carbohydrate ceiling. Research supports replacing 30 to 60% of hourly calorie expenditure with carbohydrate during endurance events lasting more than 90 minutes. If your watch says you burn 800 calories per hour at race pace, you're targeting roughly 60 to 120 grams of carbohydrate per hour — and your gut tolerance determines where in that range you land.

Turning three numbers into a plan

Now you have heart rate at race pace, estimated sweat loss, and calorie burn per hour. Here's what to do with them.

Step 1: Set your fluid target

Take your sweat loss estimate and aim to replace 60 to 80% of it per hour. Replacing 100% is unnecessary and risks hyponatremia if you're drinking more than you're losing. For most marathon runners, this lands between 400 and 700 ml per hour.

Map this against the aid station spacing for your race. If aid stations are every 3 km and you need 600 ml per hour at a 5:30/km pace, that's roughly 100 ml at every other station — not every station. Knowing your number means you can plan exactly when to drink and when to skip.

Step 2: Set your carbohydrate target

Use your calorie burn to calculate your carb need. A reasonable starting point: 0.8 to 1.0 grams of carbohydrate per kilogram of body weight per hour for a marathon. A 70 kg runner targets 56 to 70 grams per hour.

Cross-reference with your gut training. If you've been practising fueling on long runs (and you should have been), you already know your tolerance. If 60 grams per hour felt fine, that's your floor. If you managed 80 grams with no GI issues, that's your ceiling.

Now pick your products. Each gel typically contains 20 to 30 grams of carbohydrate. A 500 ml serving of sports drink adds another 30 to 40 grams. If your target is 70 grams per hour, that could be two gels plus water, or one gel plus a full serving of drink mix. The numbers have to add up.

Step 3: Set your sodium target

If you're a salty sweater — white residue on your kit, stinging eyes, gritty skin — you likely lose more sodium per litre of sweat than average. Aim for 500 to 1,000 mg of sodium per hour in hot conditions, 300 to 500 mg in mild conditions.

Your watch can't measure sodium loss directly. But your training data gives you proxies: did you cramp on long runs despite adequate hydration? Did you feel fine in cool weather but fall apart on hot days? Cross-reference your cramping history with the temperature and heart rate data from those sessions. Patterns emerge.

Why this is harder than it sounds

If you've read this far and thought "I could do this in a spreadsheet," you're right. You could. But here's what a spreadsheet can't do:

It can't update when your fitness changes. Your sweat rate and calorie burn at race pace in week 4 of training are not the same as week 14. Your watch captured that progression, but a static plan ignores it.

It can't adjust for race-day weather. If you built your plan from August long runs at 28°C and race morning is 12°C, every number shifts — sweat rate drops, core temperature management changes, and your fluid needs could fall by 30%.

It can't account for elevation. A flat training course and a hilly race course produce different heart rate profiles at the same pace, which changes calorie burn and sweat rate.

It can't combine products intelligently. Knowing you need 70 grams of carbs per hour is useless until you've mapped it to specific products, checked the sodium content of each, verified they don't exceed your gut tolerance together, and planned which ones to carry versus pick up on course.

And you'd need to redo the entire exercise every time any of those variables change.

The missing link between your watch and your race

This is the gap that most athletes fall into. They've invested hundreds of pounds in a Garmin or Apple Watch. They've logged every run. They have more physiological data than professional athletes had access to 15 years ago.

But on race morning, they're standing at the start line with a fueling plan they copied from a blog post or a Reddit thread. Three gels, drink at every station, hope for the best.

The data to build something better is already on their wrist. What's missing is the intelligence layer that converts raw metrics into a specific, personalised, actionable plan.

Sweatr connects to your Garmin, Apple Watch, and Strava data and does exactly this. It pulls your training history, identifies your heart rate trends at race effort, estimates your sweat rate from environmental and physiological data, calculates your carbohydrate needs per hour, and builds a race-day plan with specific products, timing, and quantities.

When the weather forecast changes three days before your race, Sweatr recalculates. When your fitness improves and your heart rate at race pace drops, your plan adjusts. When you switch from marathon training to half-marathon prep, the targets shift automatically.

No spreadsheet. No manual weigh-ins. No guessing.

Your watch has been collecting the data for months. Sweatr turns it into the plan you'll actually use on race day.

What to do this week

If your race is in the next four weeks, here's the minimum version:

1. Open your watch app and find your three most recent race-pace runs. Write down the average heart rate.
2. Check the sweat loss estimate or run the weigh-in test on your next easy run.
3. Calculate your calorie burn per hour at race effort from at least two sessions.
4. Use those numbers to set a fluid, carb, and sodium target for race day.
5. Test those targets on your next long run or shakeout.

Or download Sweatr, connect your watch, and have the plan built for you in under two minutes. It pulls the data you've already logged, runs the calculations, and gives you a race-day fueling plan with specific products and timing — personalised to your body, your pace, and the conditions you'll actually race in.

Your watch did the hard work. Let your data finish the job.