Sub-4 Ironman Marathon: The Bike-Leg Energy Budget That Decides Your Run

You don’t run a sub-4 Ironman marathon on the run. You run it on the bike, hours earlier, with the power you choose in the first 20 miles. Ride a little too hard and the run bill arrives at kilometer 25.

Think of race day as one fuel tank with a fixed size. Every watt above your plan is a withdrawal you can’t put back. This post shows how to set that budget on purpose.

What a sub-4 Ironman marathon actually takes

Four hours over 42.195 km works out to 5:41/km (9:09/mi). That’s the average for the whole run, including aid-station walking and any toilet stop. In short: you can’t just hold 5:41 and hope. You need to run a bit faster than that to cover your stops.

Most athletes aim for 5:30 to 5:35/km (8:51 to 8:58/mi). That banks 3 to 5 minutes. Then there’s the fade. Almost nobody runs even splits at the end of an Ironman, so the table below shows what each finish demands under a gentle fade and a hard one.

Pure arithmetic, no study behind it. With a 10% fade, a 4:00 marathon means leaving the bike ready to run 5:25/km. Translation: the slower you fade, the easier your first 10 km can be.

Bike intensity factor targets for the Ironman

Your FTP (Functional Threshold Power) is the highest power you can hold for about an hour. Your intensity factor (IF) is your average power for the ride divided by FTP. An IF of 0.70 means you averaged 70% of your hour-power across the whole bike.

Coaching sources put most age-groupers between 0.65 and 0.75. The sweet spot is 0.68 to 0.72. Strong runners often sit at the low end, and strong cyclists at the high end. Pros at Kona ride near 0.80. One Kona age-group winner reported by TrainingPeaks rode 0.70.

Keep your variability index at 1.05 or lower. That’s your normalized power divided by your average power, and surges push it up. In plain words: ride like a metronome, not a heartbeat monitor.

Steady is faster than fast-then-slow.

What overbiking really costs

You’ll read that riding 3 to 5% above target power costs 20 to 30 minutes on the run. We looked for the data. It doesn’t exist. No peer-reviewed study we found links bike intensity to Ironman marathon fade. Treat that number as folklore.

Here’s what we can model. Start a 4:00 marathon at 5:25/km. A 10% second-half fade finishes in 4:00. A 20% fade finishes near 4:11. A 30% fade lands near 4:23.

So 20 to 30 minutes is possible, but only if your pace falls apart by about 30%. A short effort budget overdraft rarely does that. A big one does.

The bike-leg energy budget: carbs in, carbs out

Your body stores a limited amount of carbohydrate, and an Ironman bike burns through it faster than most people expect. Exact numbers vary by person, so we won’t pretend to give you one. What you can control is how much you eat.

For events over 2.5 hours, Jeukendrup’s review supports up to 90 g of carbs per hour from a glucose and fructose blend. Mixing the two sugars raises how much you burn from your food by up to about 65% versus glucose alone. A single sugar tops out near 60 g/h. In short: two sugar types use two doors into your bloodstream, not one.

Trained cyclists have taken in about 120 g/h in lab work. We found no Ironman trial testing more than 90 g/h, so treat 120 as an experiment for training rides.

The harder you ride, the more of your fuel comes from carbs. The same plan that works at 0.68 IF can leave you empty at 0.75.

Training load: the bike’s bill in one number

A simple way to compare rides is bike training load: IF squared, times hours, times 100. Most age-groupers land between 250 and 285.

Translation: a fast ride at 0.75 for 5 hours costs about the same as a steady ride at 0.68 for 6. Which one leaves you with a better run? The steady one, for most people.

Dehydration: what 3% loss does and doesn’t cost

A warm-weather Ironman study by Laursen and colleagues found athletes lost about 3% of body mass on average. Well-trained triathletes tolerated that without their core temperature running away. A South African Ironman study by Sharwood and colleagues found big weight loss wasn’t tied to more medical problems. Weight gain, on the other hand, tracked with low blood sodium.

That doesn’t mean skip drinking. It means a scale reading of minus 3% is not an emergency, and plus 1% isn’t a win. Drink to your own sweat rate, not a rigid number.

Durability: why your fresh marathon time lies

A marathon PR from a fresh start overstates your Ironman run. Four to six hours of riding changes your legs. In plain English: fresh-legs tables assume you start the run rested, and you won’t be.

Think of your legs as a phone battery. A fresh marathon starts at 100%. An Ironman marathon starts at whatever the bike left you.

Case study: Dana’s two Ironman runs

Take a rider I’ll call Dana, a fictional 41-year-old with a 250 W FTP. In her first Ironman she rode an IF of 0.76 (190 W) and split 5:25 on the bike, a training load of about 313. She walked from kilometer 22 and ran 4:20.

The next year she held 0.70 (175 W). The bike took 5:42, a load of about 279. Her marathon went 3:56, a 24-minute gain for 17 minutes on the bike. Net: seven minutes faster, and she felt better doing it.

Set your power ceiling from your run goal

Work backward. Pick your run target, check your fade in long brick sessions, and choose the IF that leaves you that margin. If your long run drifts 5% after a hard ride, a 4:00 marathon asks for roughly 5:33 per km to start.

AthleteOS’s AI coach does this inside your training plan, using your own training data and race goal instead of generic fresh-legs tables. You can try it with the 14-day trial.

For more on race-week prep, see our 70.3 taper guide and how Zone 2 builds the aerobic base. To ride steady instead of spiky, read why even bike pacing protects your run, and see the 10% decay rule for threshold pace.

Your marathon pace is already decided by mile 20 of the bike.