The same run costs more on some days than others
There is a kind of run where nothing is wrong and everything is hard. The route is one you have done twenty times. You slept, more or less. Nothing hurts. And from the first hundred metres your legs feel like they belong to somebody heavier and older, and the pace you held easily last week has turned into a project.
Most people finish a run like that and reach straight for the obvious conclusion. I have lost fitness. I was never really fit. The last two months must have been luck. It does not arrive as a thought you can argue with. It arrives as a fact, which is what makes it so good at ending things.
The price changes, and nobody tells you in advance
The same distance costs a different amount every time you buy it. A short night. A meal eaten too close to the start, or not eaten at all. Air that sits on you. A fortnight at work that has been quietly grinding you down. An argument you have not finished having. Any of it turns up in your legs.
You do not keep one compartment for your life and another for your running. There is just you, carrying whatever you are carrying, out on the same stretch of road you were on last Sunday. The road has not changed. You are simply arriving at it as a different person than you were then.
What makes this maddening is the lack of a receipt. You get the cost without the itemisation, so you go looking for an explanation, and the cheapest explanation available is always yourself.
How you feel and how fit you are move at different speeds
Fitness is slow. It gets built across months and it leaves across months. Nothing you did or failed to do on a Tuesday has quietly undone it by Wednesday, however convincing Wednesday feels.
How you feel is fast. It shifts hour to hour and responds to everything. So when you read today's heaviness as a measurement of your fitness, you are using a jumpy instrument to read a slow one. You will get a different answer every time you check, and the frightening answers will always feel more credible than the good ones.
Watch what people actually do with this. A good day gets filed as normal and forgotten by the evening. A heavy day gets filed as the truth finally coming out. Same body, same person, four days apart, and only one of the two readings is ever believed.
Change the run, not the verdict
When a run is heavy, the useful thing to change is the run. Slow down until it is manageable, even if the number on your wrist is embarrassing. Cut it short, or walk part of it, or turn the session you had planned into an easy half hour and let that be the whole of it.
People treat that as caving in. It is closer to knowing what you are doing. The plan was written on a different day, by someone with less information than you have now. You are standing in the actual weather. You are allowed to answer the day in front of you rather than the one you imagined on Sunday night.
What you decide it counts as matters far more than what you did. It counts. A slow, shortened run is a run. You kept the appointment, and you got some honest practice at the thing you will be doing for years, which is adjusting to a body that has good weeks and bad ones.
Why the heavy ones are worth finishing anyway
Anyone who has been running a long time has a private collection of sessions that felt awful. They rarely mention them, because there is nothing to say. You went out feeling terrible, you moved slowly, you came home. Nothing happened that anybody could photograph.
Those runs do teach you something, though, and it takes doing rather than reading. You learn from experience that feeling bad at the start has no authority over what you do next. Once you have proved that to yourself twenty times, a heavy day stops being a decision point. It becomes weather. You dress for it and you go.
You will get another one soon, probably sooner than you would like. It will not mean anything about you then either. Go slowly, finish it however you need to finish it, and let Thursday turn up with its own weather.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.