Why Your Actual Race Pace Didn't Match Your Training Pace
The most common causes are starting too fast due to race-day adrenaline (leading to a slower second half), weather conditions different from typical training conditions, course terrain and elevation not present in training runs, and the effects of a pre-race taper changing how the body feels compared to a normal training week.
A race pace that diverges from training pace — in either direction — is extremely common, and it's usually explainable by one or more specific, identifiable factors rather than a sign training didn't work.
Starting too fast
Race-day adrenaline commonly leads to a faster-than-planned start, which can feel great in the early miles but often results in significant slowing in the back half of a race as fatigue outpaces the body's ability to sustain that early pace — a classic and extremely common pacing mistake, even among experienced runners.
Weather and course differences
Heat, humidity, wind, and course elevation all affect sustainable pace, sometimes significantly — a race run in noticeably hotter or hillier conditions than typical training runs will naturally produce a slower pace than flat, cool training conditions predicted, independent of actual fitness level.
Taper effects
A pre-race taper (reduced training volume in the days or weeks before a race) is designed to leave the body fresher and more capable of a faster pace than typical training weeks — some runners find their legs feel noticeably different (sometimes better, sometimes oddly flat) during a taper, which can affect how race pace compares to recent training paces in either direction.