The question, read

Why was my long run slower than last month?

Most slow long runs are not decline. One slow run, on its own, is weak evidence. Weather alone moves easy pace by half a minute a kilometre. Here is the read, in the order the causes are likely.

1. Fatigue you have not absorbed yet

Look at the ten days before the run, not the run itself. Volume up, two hard sessions close together, short sleep — any of these leaves the block sitting on your legs.

This is the ordinary cost of training, and it shows up on the watch before it shows up anywhere else. A pace drop in a heavy week says your body is working, not that your fitness is going backwards. Coaches expect this and read through it; you can too.

2. Conditions you are comparing unfairly

Above about 15°C, easy pace slows by 10 to 20 seconds a kilometre for every 5 degrees warmer, and more in humidity. Wind, hills, trail instead of road, and running them alone instead of in a group all land on the same line of the log.

Two long runs a month apart are rarely the same run. Before you call it decline, put the two mornings side by side: temperature, surface, and who you were with. Often the whole gap is there.

3. A real trend

Real decline is slower to show and harder to fake. It takes weeks: long runs drifting over three or more outings, easy runs slowing with them, resting heart rate creeping up.

One number never settles it. The honest comparison is pace at the same effort, not pace on the clock. If that has moved for three weeks, the fix is usually a down week, not a harder one.

What to do this week

If the ten days before were heavy, take 30% off the week: same frequency, shorter and slower, and let the long run be short. If the weather explains it, change nothing and log the temperature next time.

If it has been three weeks of drift, cut the volume and book an easier week, then re-read. And if none of it settles, that is exactly the question a second read of the whole block is for.

The part a watch cannot tell you

Strava and Garmin will happily graph both runs. Neither names the cause. That takes reading the whole block around them: volume, session mix, and the weeks in between.