The Beginner's Guide to Not Blowing Up on the Run
Every triathlete has a story. Usually, it goes something like this: "I felt amazing on the bike. I was right there with the lead group, but then the run happened and my legs just completely died." What that athlete experienced wasn't a lack of running fitness. It was the cumulative cost of pacing choices made on the bike leg, about 45 minutes earlier.
This is the hard truth that separates age-group triathletes who cross the finish line smiling from those who shuffle across asking themselves why they ever signed up. The bike leg doesn't just contribute to your triathlon time; it fundamentally determines whether your legs work on the run. Unlike swimming, where effort is relatively short and manageable, the bike is long enough and intense enough that pacing decisions can have massive downstream consequences.
The good news? Understanding the science behind bike pacing and learning to apply it to your own racing can be a complete game-changer. Let's dive in.
Why the Bike Leg Determines Your Run
Your body has finite resources; glycogen stores, aerobic power, and the capacity to buffer metabolic byproducts all determine what's left in the tank for the run. If you deplete your glycogen or accumulate too much lactate and hydrogen ions on the bike, your run performance doesn't suffer a little bit - it can suffer catastrophically.
Consider two athletes racing a sprint triathlon with identical running fitness. Athlete A bikes conservatively at 85% of FTP for 20 minutes, finishes with decent glycogen stores and minimal metabolic debt. Athlete B rides aggressively at 110% FTP for the same 20 minutes, feels strong, and is convinced they're having the race of their life.
However, their 5-km run times will be dramatically different. Athlete A might run a 22-minute 5K pace. Athlete B might be looking at a 30-minute pace, or worse, they might not be able to sustain any pace at all - the dreaded "blow up" after the bike. The difference is that Athlete B's body is already in metabolic crisis before they even put on their running shoes.
This relationship is especially pronounced in longer distances. In a 70.3, your bike leg is roughly 2-3 hours of sustained effort. Pacing mistakes there aren't minor - they're compounding interest on a physiological debt that you'll be paying off for the next 10+ miles on foot.
The Science of Blowing Up: Why It's Worse Than You Think
"Blowing up" in a triathlon isn't poetic. It's metabolic. And the mechanism is more complex than simply "running out of energy."
When you exceed your sustainable power output, your body shifts increasingly toward anaerobic metabolism. This produces lactate and hydrogen ions as byproducts, which accumulate in your blood and muscles faster than your aerobic systems can clear them. At the same time, you're depleting glycogen stores at an accelerated rate. Glycogen isn't just fuel - it's also needed to buffer acid in your muscles.
Best Bike Split models this exact effect segment-by-segment, giving you terrain-appropriate power targets that minimize variability and preserve your run legs.
Common Pacing Mistakes (And Why They're So Tempting)
Most triathletes aren't trying to blow up on the bike. They make pacing mistakes because certain patterns feel right in the moment, even when they're aerobically catastrophic.
The Hot Start
The most common mistake is starting too hard in the first 30 minutes. You've just come out of the water. You're fresh. The adrenaline is still coursing through your system. Everyone around you seems to be hammering, and your brain is screaming that you need to keep up. You ride the first 15-20 minutes at FTP, feeling great, feeling in control. By minute 25, you realize you've made a terrible mistake - you're already breathing hard and you've got 2 hours left.
The problem is that the first 30 minutes of the bike feel disproportionately easy, even when your power output is unsustainably high. Your glycogen is full. Your lactate clearance is still working well. Your muscles haven't had time to accumulate fatigue signals. This creates a dangerous illusion of sustainable pace. Then, around minute 30-40, the reality sets in. You've already triggered significant glycogen depletion and accumulated a lactate debt. By the time you settle into a sustainable pace, you've already done metabolic damage that will haunt your run.
Surging on Hills
Hills are where variable-power mistakes happen most frequently. A hill appears. You feel a shift in your bike dynamics. Almost automatically, you increase power to maintain speed - maybe not consciously, but because the hill demands it. You drop two gears and hammer. The hill lasts three minutes. You summit feeling accomplished, then ease back on the flats.
The problem is that those three minutes at 140% FTP cost far more than three minutes at 90% FTP plus some anaerobic debt. The surge created a lactate spike your body couldn't immediately clear. You've now accumulated metabolic waste that will take 10+ minutes to clear, and you've burned through glycogen reserves. If you do this four or five times across a 2-hour ride, those surges have created a collective lactate and glycogen debt that completely changes your run.
Chasing Other Athletes
This is perhaps the most insidious mistake because it combines ego, real-time feedback, and the competitive atmosphere of a race. Someone passes you. Without conscious thought, you accelerate to match their pace. A few minutes later, you do the same thing to someone else. Over the course of a ride, you've created dozens of unplanned micro-surges - each one a variable-power spike, and each one accumulating lactate and burning expensive glycogen.
Going By Feel and Ignoring Power
"I just go by feel" is something you hear a lot from age-group triathletes, usually followed by stories about how they blew up on the run. The problem is that feel is a horrible pacing metric. Your perceived effort is influenced by your emotional state, fresh legs early in the race, novelty, and a dozen other factors that have nothing to do with your sustainable metabolic output. Power, by contrast, is objective and real. It's telling you the actual mechanical work your body is performing, which is what actually determines your glycogen burn and lactate production.
Heart Rate Is a Trailing Indicator; Power Is Real-Time
This distinction matters for race execution, so it's worth spending a moment on it.
Heart rate is a lagging indicator of effort. When you spike your power suddenly, your heart rate doesn't immediately jump to match. It takes 20-40 seconds for your cardiovascular system to respond, and that lag is longer during variable-power efforts. This is why heart rate-based pacing can create a false sense of control. You're riding at 120% FTP, but your heart rate is still sitting at the heart-rate equivalent of 95% FTP, so you feel like you're in control. You're not. Your muscles know you're not. Your lactate levels know you're not. Your heart rate is only one part of the story.
Power, by contrast, is real-time. When you hit 120 watts, you're doing 120 watts immediately. There's no lag, no illusion, no delayed feedback. Power tells you exactly what your body is asking of your aerobic and anaerobic systems right now, not 30 seconds from now.
For race execution, this means power is the metric you should be using to guide your pacing decisions. Heart rate can be a useful secondary metric to ensure you're not overreaching relative to your aerobic threshold, but it shouldn't be your primary guide. If your power meter shows you surging but your heart rate hasn't caught up yet, power is going to give a more accurate picture of your effort.
Practical Pacing Frameworks: FTP-Based Targets by Distance
So, what should you actually be targeting? The answer depends on the distance of your race and, to some degree, your fitness level and racing experience.
| Race Distance | Bike Power | Notes |
|---|---|---|
| Sprint | 95-105% of FTP | Shorter is hotter. You can sustain a higher intensity for 20-30 min than for 90+ min. |
| Olympic | 85-95% of FTP | The sweet spot for most age-groupers. Sustainable for 60 minutes while preserving run performance. |
| 70.3 (Half Ironman) | 75-85% of FTP | Longer duration demands lower intensity. Most age-groupers should target the lower end of this range. |
| Ironman | 68-78% of FTP | Ultra-long distance requires ultra-conservative pacing. The best IM performers often ride at the low end here. |
These ranges assume you've established your FTP through testing (a 20-minute all-out effort, with the average power from the last 20 minutes multiplied by 0.95). They also assume you're an age-group triathlete with typical power output relative to body weight and typical run fitness.
The key principle here is that longer distances demand lower intensities. This isn't just because you can't sustain high power for long periods. It's because the metabolic cost of running even a modest distance (5K on a sprint triathlon, 10K on an Olympic, 13.1 miles on a 70.3, 26.2 miles on an IM) means you need to arrive at the run with intact glycogen stores and minimal lactate debt. The longer the run, the further down on the intensity scale you need to go.
How Best Bike Split Accounts for Course-Specific Demands
This is where the physics meets the road. Every course is different. A flat time-trial demands different pacing than a hilly circuit. A windy coastal ride demands different pacing than a sheltered valley ride. An IM course with a 2,000-foot climb demands different pacing than a pancake-flat IM course.
Best Bike Split's approach uses physics and physiology-based power modeling that accounts for the actual course profile, wind patterns, your equipment, and your fitness. Instead of giving you a single target power, the tool creates a course-specific power plan that tells you what power you should be targeting on each segment.
For a rolling 90-minute Olympic course, BBS might suggest:
- Flats and descents: 170W
- Moderate climbs: 200W
- Steep climbs: 215W
Best Bike Split creates these segment-by-segment targets automatically using your FTP (functional threshold power), CdA (coefficient of drag), weight, and the actual course profile - accounting for the physics that make variable pacing so costly.
This approach prevents the variable-power mistakes that happen when you're reacting to terrain without a plan. Instead, you have a proactive plan driven by science. When you see a climb coming, you already know what power you should target. You're not reacting. You're executing.
The same principle applies to wind. A headwind section might call for slightly lower power (accepting that you'll go slower into the wind). A tailwind section might allow slightly higher power. The key is that these adjustments are part of an integrated plan designed to minimize variable-power spikes while hitting a target average.
Bringing It All Together: Your Race Day Pacing Plan
So what does this look like on race day?
You should arrive at your race knowing three things:
- Your FTP (established through a reliable testing protocol)
- Your course-specific power targets (ideally generated by Best Bike Split, accounting for terrain, wind, and equipment)
- Your power cap (the maximum intensity you'll sustain regardless of what's happening around you)
When the gun goes off and you transition to the bike, you execute your plan with discipline. You ignore the athletes around you. You ignore how fresh your legs feel at the 5-minute mark. You ignore the temptation to surge on that hill. You execute your power cap, you aim for your segment-specific targets, and you accept that your job on the bike is not to race as hard as possible. Your job is to preserve your aerobic capacity and glycogen stores for the run.
This isn't the most exhilarating way to describe race execution, but it's the way that works. It's the way that turns a bike ride that feels great at 45 minutes into a run that feels great at 15 kilometers. It's the way that separates the athletes who cross the finish line and meet their race-day goals from the athletes who shuffle across the finish line, or don't cross at all.
Start Racing Smarter, Not Just Harder
Triathlon is hard enough without sabotaging yourself on the bike leg. The good news is that pacing is something you can completely control. You don't need more fitness. You don't need more talent. You need a plan and the discipline to execute it.
That's exactly what Best Bike Split is designed to do. Instead of guessing at your bike pacing, build a course-specific power plan that accounts for terrain, wind, and your actual equipment. Get segment-by-segment guidance that prevents the variable-power mistakes that kill your run. Know exactly what you should be targeting on every part of the course - and more importantly, know why you're targeting that number.
The difference between an athlete who blows up on the run and an athlete who runs strong isn't effort. It's execution. It's the discipline to hold back on the bike so you can attack on the run. And it's the planning that makes that discipline possible.
Ready to stop guessing at your bike pacing? Sign up for a free account today to create your course-specific power plan with Best Bike Split today and race smarter on every course.
