Why You Faded in the Last 10K (and How to Fix It)

If you slowed down hard in the last 10K, it was almost always one of four things: you started too fast, you underfueled, your long-run volume was too low, or you lack durability — the ability to hold pace when tired. The good news: all four are fixable.

The table below matches what you felt to the likely cause and the fix.

Symptom	Likely cause	Fix
Sudden crash near mile 20	Started too fast	Run even or slightly negative; trust your projection
Empty, lightheaded, no energy	Underfueled	Take 30–60 g carbs per hour; practice it in training
Legs gave out, never felt fit enough	Long-run volume too low	Build weekly miles and your longest run over weeks
Paced and fueled well, still faded	Low durability	Add marathon-pace segments late in long runs

The quick answer

A late fade is not bad luck. It has a cause, and usually it is one of four. Find yours in the table above, then read the section below to be sure.

The most common cause by far is a fast start. The second is fuel. The third and fourth are about training — how much you ran and how well your fitness holds up when tired.

Fix the cause, and your next race finishes strong.

The four common causes

1. You started too fast. This is the top reason runners fade. When you run faster than goal pace early, you burn your stored carbohydrate too soon. That store — called glycogen — can run dry around mile 20, which is "the wall" (Rapoport, 2010). Then your pace crashes.

2. You underfueled. A marathon needs fuel during the race, not just before. Research on marathon runners suggests taking in 60 to 90 grams of carbohydrate per hour for longer efforts (Jiménez-Alfageme et al., 2025). Beginners can start near 30 to 60 grams. Skip the fuel and the tank empties early.

3. Your long-run volume was too low. The marathon is built on endurance. A big study found that more weekly miles and a longer long run go with faster, stronger races (Fokkema et al., 2020). If your longest runs were short, the last 10K will feel like new territory.

4. You lack durability. Durability is how well your fitness holds up when you are already tired. Two runners with the same fresh speed can finish very differently — the one who resists fatigue stays strong (Maunder et al., 2021). Low durability means you fade even when pacing and fuel were fine.

How to tell which one was you

Think back to the race. Your memory holds the clue.

• Did you feel great early, then crash suddenly? That points to a fast start. You were ahead of pace and paid for it.
• Did you feel empty, dizzy, or weak — not just tired legs? That is fuel. You needed more carbs during the race.
• Did your legs feel like they had never done this distance? That is volume. Your long runs were too short.
• Did you pace smart and fuel well, but still slow in the last few miles? That is durability. You need to train tired.

Most fades fit one of these. Some are a mix. Fixing the fade usually means smarter pacing plus fast-finish long runs (McMillan Running).

Build a projection that accounts for fade

The first fix is the easiest: stop chasing a time you cannot hold. Enter a recent race and let PIRX project a realistic finish — free, no signup, no watch. A goal that fits your fitness is a goal you can pace without fading.

PIRX's proprietary machine-learning prediction engine learns your style instead of using one fixed formula. It shows a supported range, so you pick a target you can hold to the line. Inside the full PIRX app, drivers break down which factors — training volume, recovery, and durability — are holding your projection back, so you know exactly what to work on.

This is what makes PIRX different. Strava tells you what you did. Garmin shows your data. PIRX tells you why it happened and what's next — with validated prediction accuracy of 98% across PIRX users.

New here? Start with the complete guide to predicting your race time. Want to pace it right next time? Read how to pace a marathon. Building your endurance? See what pace your long run should be. When you are ready, get a realistic projection below.

Sources

• Rapoport, B.I. (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology. (Models when glycogen runs out — "the wall.")
• Jiménez-Alfageme, R. et al. (2025). Nutritional intake and timing of marathon runners. Sports Medicine - Open. (Carbohydrate intake ~60–90 g/h for long events.)
• Fokkema, T. et al. (2020). Training for a (half-)marathon: training volume and longest endurance run related to performance. Scandinavian Journal of Medicine & Science in Sports.
• Maunder, E., Seiler, S. & Mildenhall, M.J. (2021). The importance of 'durability' in the physiological profiling of endurance athletes. Sports Medicine.
• McMillan Running. Fixing the fade: feeling strong in the final miles.