🏁 Race Predictions: What Happens When AI Ignores Your Long Runs

Garmin Race Predictor vs Real Marathon Pace

📍 Introduction

Garmin just told me I’m capable of a 3:41:03 marathon. I nearly dropped my Maurten in disbelief.

Sure, I’ve been training hard, and yes, my fitness is improving. But if we’re going by real numbers, fatigue, fueling, and the facts, that prediction feels more like a bedtime story than a pacing plan. Let’s unpack why.

📟 Garmin’s Race Predictor: Optimism in a Watch Face

Here’s what my Garmin Forerunner 235 served up the other day:

🕓 5K: 23:10
🕕 10K: 48:03
🕘 Half: 1:46:29
🕛 Marathon: 3:41:03

Sounds nice, right?

These estimates are based on my VO₂ max and short-distance performance, but they conveniently ignore everything that makes the marathon… well, a marathon: glycogen depletion, cardiac drift, weather, fueling, fatigue, and how your legs feel at kilometer 35.

📉 Comparing Predictions vs Reality

Let’s compare what Garmin thinks I can do versus what I’ve actually run-and what a realistic estimate might look like from my recent Time Trial and training loads.

🔍 The Comparison

Distance	Garmin Prediction	Actual Race	Plus50Fit Estimate
5K	23:10	~27:30	26:30
10K	48:03	59:00	56:00
Half	1:46:29	2:25:39	2:02:00*
Marathon	3:41:03	N/A	4:17:00*

*Based on recent time trial, rTSS trends, and training progression

📊 Visualizing the Gap

Bar chart comparing Garmin race predictions, actual race times, and Plus50Fit realistic estimates for 5K, 10K, Half Marathon, and Marathon.

This chart shows what happens when algorithms get optimistic and your body keeps it honest. The Plus50Fit Predictor accounts for effort-based metrics, time trial data, Pa:HR drift, fueling strategies, and the reality of my age and training history.

🧠 The Plus50Fit Predictor: A Model With Context

Unlike Garmin’s fantasy estimates, the Plus50Fit Predictor builds from:

My July 23 Time Trial (20–30 minute peak pace data)
TrainingPeaks metrics: rTSS, hrTSS, Pa:HR, and IF (HR)
Current threshold HR
My Build Phase progression and long run consistency
Fueling and hydration protocols (hello, Maurten & SaltStick)
Strength work

This model doesn’t assume I’ll run negative splits on fairy dust and wishful thinking.

🔁 Reality Check = Progress Tracker

The Garmin predictor is a fun novelty, but the Plus50Fit Predictor is my training reality compass.

And that’s what I’ll trust as I move toward the Toronto Waterfront Marathon in October.

🧩 Final Word

The numbers on your wrist might tell one story. But the miles in your legs write another.For me? I’m not chasing Garmin’s fantasy. I’m chasing something more honest-and far more rewarding: my best marathon, built on real work.