Your Strava Data Already Knows Where You'll Bonk

You've already recorded the evidence

Every long run you upload to Strava contains a story your legs already know but your brain hasn't read yet. Somewhere between kilometre 25 and 32 of your last 30k training run, your pace drifted upward by 15–30 seconds per kilometre while your heart rate kept climbing. Your cadence dropped. You started walking through water stations instead of grabbing and going.

That pattern is glycogen depletion — the thing runners call bonking or hitting the wall. And it didn't happen randomly. It happened at a predictable point determined by your pace, your body weight, your fueling, and your sweat rate.

The data is sitting in your Strava history right now. Here's how to read it.

What bonking actually looks like in your data

Your body stores roughly 1,600–2,200 calories of glycogen in your muscles and liver, depending on your size, fitness, and how well you carb-loaded. At marathon pace, you burn somewhere between 600 and 1,000 calories per hour. Simple maths says you have 2 to 3 hours of fuel before the tank hits empty.

But the data tells you something more precise than the textbook estimate. Open your last long run in Strava — anything over 90 minutes — and look for these three signals appearing together:

1. Heart rate drift without pace increase. In the first half of your run, your heart rate and pace have a stable relationship. Your heart rate sits in Zone 2 or 3 at your target pace. Then, somewhere in the back half, your heart rate starts climbing even though your pace is the same or slower. This is called cardiac drift, and while dehydration contributes, a major driver is your body switching from burning glycogen (efficient) to burning fat (expensive). Your cardiovascular system is working harder to produce the same output.

2. Pace decay with maintained effort. You're trying just as hard. Your perceived effort hasn't changed. But your splits are getting slower. In Strava, this looks like a gradual upward slope on the pace graph starting 60–90 minutes in. If you're running with power (Stryd or Apple Watch), you'll see your pace drop while your power stays flat — the clearest sign that your engine is losing fuel, not that you're giving up.

3. Cadence drop. This is the subtle one most runners miss. When glycogen runs low, your neuromuscular system starts losing efficiency. Your turnover slows. A drop of 5–8 steps per minute from your average in the last quarter of a long run is a strong signal that you were running on fumes.

Finding your personal bonk point

Pull up your last three long runs in Strava. For each one, note the time (not the distance) where heart rate drift and pace decay both became visible. You're looking for the moment where the gap between your heart rate line and your pace line starts widening.

For most recreational marathon runners, this happens somewhere between 75 and 120 minutes. That number is your current glycogen ceiling — the point where your stored fuel runs out at the rate you were burning it.

Now compare it to your expected marathon finish time. If your glycogen ceiling is at 100 minutes and your marathon target is 4 hours 30 minutes, you have a 170-minute gap that fueling needs to cover. That's the gap between bonking at mile 16 and finishing strong.

Here's the critical insight: that gap isn't fixed. It moves based on four variables you can control.

The four variables that move the wall

1. Your starting glycogen stores. Proper carb loading in the 48–72 hours before your race can increase your glycogen stores by 25–40%. That 100-minute ceiling becomes 125–140 minutes before you even start fueling. This is why taper-week nutrition matters so much — it's not about the pasta dinner the night before. It's about 2–3 days of consistently elevated carbohydrate intake.

2. Your fueling start time. This is where most runners get it wrong. They wait until they feel hungry or tired before taking their first gel. By then, glycogen is already depleted and your body has switched to fat oxidation — a process that's hard to reverse mid-race. The research is clear: start fueling within the first 20–30 minutes of your race, before you need it.

3. Your fueling rate. Current sports science recommends 60–90 grams of carbohydrate per hour for events lasting over 2.5 hours, using a mix of glucose and fructose for better absorption. But that's a population average. Your personal carbohydrate ceiling — the maximum your gut can absorb per hour without GI distress — depends on your gut training, your body weight, and the specific products you use. If you've been training your gut with race nutrition on long runs, you may tolerate closer to 80–90g per hour. If you haven't, even 60g might cause problems.

4. Your race-day pace relative to training. This is the variable most fueling plans ignore. Running your marathon 15–20 seconds per kilometre faster than your long run pace burns glycogen significantly faster. Your Strava data shows you where the wall appeared at training pace — race pace moves it earlier. A fueling plan built from training data needs to account for the increased burn rate on race day.

Building your prediction

Here's a practical framework using your Strava data:

Step 1: Find your training bonk point. Average the time-to-drift across your last three long runs. Call this your Baseline Glycogen Window (BGW).

Step 2: Estimate your race-day BGW. If you plan to carb load properly (you should), add 20–30% to your BGW. If your race pace is faster than your training pace, subtract 10–15%. If conditions will be hotter than training, subtract another 10%.

Step 3: Calculate the fueling gap. Subtract your adjusted BGW from your target finish time. This is how long your in-race fueling needs to sustain you.

Step 4: Set your fueling rate. Divide the fueling gap by the number of feeding windows (every 20–25 minutes). This tells you how many gels or fuel servings you need and when to take them.

Step 5: Start early. Your first fuel should come at 15–20 minutes, well before you feel any need for it. By the time your body signals hunger, you're already 20–30 minutes behind on fuel.

For a 4-hour 30-minute marathoner with a training BGW of 100 minutes and proper carb loading, the adjusted BGW might be around 120 minutes. The fueling gap is 150 minutes. Starting at minute 15 and fueling every 20 minutes, that's roughly 7–8 fueling windows — which maps to around 60–70g of carbohydrate per hour if you're using standard gels.

Why generic plans fail (and data-driven ones don't)

The reason "take a gel every 45 minutes" fails so many runners is that it ignores every variable that matters. A 55kg runner targeting a 3:30 marathon and a 90kg runner targeting a 5:00 marathon have completely different glycogen stores, burn rates, sweat rates, and absorption capacities. The same gel schedule cannot possibly work for both.

Your Strava data contains the inputs that make a plan personal. Your heart rate tells you how hard your body is working. Your pace tells you how fast you're burning fuel. Your historical long run data tells you where your specific body starts running out of glycogen at your specific fitness level.

The missing piece for most athletes is turning that data into a plan. You can do the manual analysis described above — and for your next race, you should, because understanding the mechanics makes you a better athlete. But the maths gets complex when you factor in sweat rate, conditions, body weight, and product-specific carbohydrate profiles.

Sweatr pulls your Strava activity data alongside your Apple Watch or Garmin metrics and builds this calculation automatically. It finds your personal glycogen ceiling, accounts for your race-day conditions, and generates a minute-by-minute fueling plan mapped to real products. No spreadsheets, no guesswork.

The test before race day

You have long runs left before your taper begins. Use one of them as a prediction test:

1. Run your next long run at target race pace for the middle 60–90 minutes.
2. Fuel with your planned race-day products at your planned intervals.
3. After the run, check Strava for the drift pattern. Did heart rate stay coupled with pace longer than previous runs? Did your cadence hold? Did the bonk point move later?

If yes, your fueling plan is working. If the drift pattern appeared at the same time or earlier, you need more fuel, earlier fuel, or different products.

One data point changes everything. The wall isn't a mystery — it's a number you can find in data you already have.