Why I Coach by RPE First (And Why Your Watch Shouldn't Be in Charge)

By Ascent Ultra Coaching

If you've spent any time around runners, you've probably heard the debate:

Should you train by heart rate or by pace?

The answer is... Both.

But neither should be in the driver's seat.

At Ascent Ultra Coaching, I coach primarily using Rate of Perceived Exertion (RPE) because it's the one metric that never loses touch with what's actually happening inside your body.

Your Watch Doesn't Know Everything

Modern GPS watches are incredible tools. They give us heart rate, pace, elevation, cadence, and more data than ever before.

But every one of those metrics has limitations.

Heart rate tells us how hard your cardiovascular system is working.

Pace tells us how fast you're moving.

Neither one tells the whole story.

What they don't measure is how your body is actually experiencing the effort.

That's where RPE shines.

Why Heart Rate Isn't the Whole Answer

Heart rate is an excellent tool, but it's influenced by far more than just how fit you are.

Your heart rate can climb because of:

• Heat and humidity

• Altitude

• Dehydration

• Caffeine

• Poor sleep

• Stress

• Fatigue from previous training

  
  

Imagine running threshold intervals on a 95°F afternoon.

Your heart rate will likely be much higher than it would be on a cool spring morning, even if you're producing the exact same training stimulus.

If you slow down simply to keep your heart rate in a predetermined zone, you may actually undertrain the workout you were trying to accomplish.

The goal isn't always to chase a number on your wrist.

The goal is to achieve the right physiological adaptation.

Pace Isn't Perfect Either

Pace works great on flat roads.

Ultra runners rarely race on flat roads.

Steep climbs, technical trails, mud, rocks, snow, altitude, and long descents all make pace a poor representation of effort.

A 12-minute mile can feel effortless on smooth terrain.

That exact same pace on a steep, rocky climb might require near-threshold effort.

Your watch only sees pace.

Your body knows the difference.

The Missing Piece: RPE

Rate of Perceived Exertion combines everything your body is experiencing into one simple question:

"How hard does this actually feel?"

Your brain is constantly processing information that your watch can't measure, including:

• Muscle fatigue

• Breathing rate

• Mechanical stress

• Cardiovascular strain

• Overall energy levels

  
  

RPE brings all of those signals together.

That's why experienced ultra runners can pace themselves incredibly well without staring at their watches every thirty seconds.

They've learned to trust their internal effort.

Why This Matters Even More in Ultras

During a road marathon, heart rate and pace often stay reasonably aligned.

In an ultra marathon...

Not so much.

As the miles add up:

• Your legs become fatigued.

• Your running mechanics change.

• Terrain constantly varies.

• Weather changes.

• Nutrition and hydration fluctuate.

• Elevation affects your breathing.

  
  

Late in a race, your heart rate might actually stay relatively low while your legs are completely cooked.

Your cardiovascular system isn't necessarily the limiter anymore.

Your muscles are.

If you're only watching heart rate, you might think you're doing fine while your body is telling a completely different story.

How We Use RPE at Ascent Ultra Coaching

RPE is the foundation of nearly every run we prescribe.

Heart rate and pace are valuable feedback tools, but they support the workout, they don't dictate it.

Here's the general effort scale we use:

RPE 2 - 3 Easy

Recovery pace. Conversation is effortless. This is where most easy runs should live.

RPE 4 - 5 Aerobic

Comfortably steady. You can still talk in short sentences. This is where the majority of your weekly mileage belongs.

RPE 6 - 7 Moderate

Breathing becomes noticeably heavier. Conversation becomes difficult. Used for steady efforts and some progression runs.

RPE 8–9 Threshold

Hard but controlled. Only a few words at a time. Perfect for threshold intervals and sustained climbing efforts.

RPE 10 – Maximum

Very hard. Reserved for short VO₂ max intervals, hill sprints, and maximal efforts.

Learn to Calibrate Your Body

One of the most overlooked skills in ultrarunning isn't fitness.

It's awareness.

Every easy run gives you an opportunity to compare:

• How does this effort feel?

• What does my heart rate say?

• What pace am I holding?

  
  

Over time, those signals become calibrated.

Eventually, you won't need your watch to tell you that you're going too hard.

You'll already know.

That's an enormous advantage on race day.

Final Thoughts

Technology is a fantastic training partner.

It's just not the coach.

Heart rate tells you about your cardiovascular system.

Pace tells you about your speed.

RPE tells you how your entire body is responding.

When all three agree, you're in a great place.

When they don't, trust the one that's integrating everything your body is experiencing.

Because in ultrarunning, your greatest pacing tool isn't on your wrist.

It's between your ears.

Train Smarter with Ascent Ultra Coaching

At Ascent Ultra Coaching, every training plan is built around science, but it's personalized to your body, your goals, and your life. We combine RPE, heart rate, pace, strength training, and real-world trail experience to help you train with purpose, not just chase numbers.

Whether you're preparing for your first 50K or your next 100-miler, our goal is simple: help you become a stronger, smarter, and more confident ultrarunner.