Your body is not the thing you are fighting
Listen to the way you talk about your body when you are trying to get fit again. Most people describe a management problem. Something to be dragged out of bed, whipped into shape, got under control, burned off after a heavy weekend. The vocabulary belongs to a difficult subordinate, and you use it about the only body you have.
You did not invent that language. It came with the culture around fitness, where the body is a project with a deadline and you are the one holding the clipboard. It sounds like motivation. It behaves like something else entirely.
The words give it away
Watch for the moment a run becomes a punishment. You ate badly, so now you owe a few miles. You skipped Tuesday, so Wednesday has to hurt to make up for it. The run is no longer something you do because you like the person it makes you. It is a sentence being served.
Watch for earning, too. Earning your dinner, earning a day off, earning the right to sit down in the evening. Once food and rest have to be bought with exercise, you have set up a small economy inside your own life in which you start every day in debt.
None of it is unusual. Almost everyone who comes back to running after a long gap arrives carrying some version of it, often without noticing, because the language is so ordinary that it sounds like common sense.
What it costs you on a Tuesday evening
When you are at odds with your body, every run turns into an argument you have to win. Effort stops being effort and starts being compliance. Heavy legs stop being information and start being insubordination. You go out at half past six already braced for a fight with the thing that is going to carry you round.
That approach does work for a while. Most people can bully themselves into six weeks of anything. What it does not survive is a bad month, and bad months are guaranteed. The moment the schedule breaks, an adversarial system has only one move available: more discipline, applied harder, which is exactly what you have least of when work is difficult or you are not sleeping.
There is a quieter cost as well. If your body is the opposition, you stop listening to anything it tells you, because listening feels like losing. You end up with no way to tell tiredness from laziness, or a bad day from a warning, since all of it gets filed under weakness and overruled.
It is not withholding anything from you
Your body is not refusing to be fitter. It is not sitting on some improvement it would hand over if only you were stricter with it.
Legs feel heavy because of what you asked of them recently, or how you slept, or the sort of week you have had. Progress arrives slowly because adaptation is slow, and it was slow for everyone who has ever done this. That is not defiance. That is just the pace at which the thing works, and no amount of contempt speeds it up.
Meanwhile the body you are annoyed with got you out of the door tonight, and it has been quietly recovering from every session you gave it without being asked or thanked. It is holding up its end of this while you keep score against it.
Being on the same side
This is not a request to love your body, which is a big ask for anyone mid-attempt at changing it. Something plainer will do. Treat it as a colleague you are stuck working with for the rest of your life, and act accordingly.
In practice that is mostly a change of question. Instead of asking what you can force out of yourself tonight, ask what today can reasonably do, and go and do that. Instead of finishing a run by totting up where it fell short, notice what it actually managed. Instead of describing yourself as lazy, describe the specific situation: you slept badly, or the week was long, or you went out too fast and paid for it.
Precision is kinder than judgement, and it is more useful, because you can respond to a real circumstance and you cannot respond to a character flaw.
You will still have runs that hurt. Being on good terms with your body does not make effort comfortable, and the hard parts stay hard. What changes is who you are doing it with. The run stops being something you inflict on yourself and becomes something the two of you go out and get through together, which is a far easier arrangement to keep going for years.
You are going to be in this body for the rest of it. It is worth being on speaking terms.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.